Opening your child's first investment account: a step-by-step guide to custodial index fund investing

You don't need a lot of money, a financial advisor, or a complicated strategy. Opening an investment account for your child takes about 20 minutes at Fidelity or Vanguard. Here's exactly what to do, what to buy, and what happens as the child grows up.

UTMA vs. UGMA: which account type?

Custodial brokerage accounts for minors come in two forms: UTMA (Uniform Transfers to Minors Act) and UGMA (Uniform Gifts to Minors Act). Both work the same way for practical purposes — a parent or grandparent opens and manages the account as custodian, and the assets transfer to the child at a specified age.

The differences are minor for most families:

• UTMA is available in most states and allows a broader range of assets (securities, real estate, patents, other property)
• UGMA is available in all states but historically restricted to securities only
• For a family investing in index funds, both work identically
• The transfer-to-child age is typically 18–21, but varies by state. UTMA allows some states to extend this to 25

For most parents, the recommendation is simple: open a UTMA custodial brokerage account at Fidelity or Vanguard and invest in a broad index fund.

Which brokerage: Fidelity vs. Vanguard

Fidelity is the easiest option for most parents. The online application is fast, there's no minimum to open, and you can invest in FZROX — the Fidelity ZERO Total Market Index Fund — which has a 0.00% expense ratio. Zero. The fund tracks the entire U.S. stock market. You cannot hold FZROX at any other brokerage (it's Fidelity-exclusive), but for a custodial account you intend to hold at Fidelity long-term, this is ideal.

Vanguard is equally strong. Their equivalent funds are VTSAX (admiral shares, $3,000 minimum) or VTI (ETF version, no minimum — buy in dollar amounts). Expense ratio: 0.03%. Vanguard's platform is slightly less modern than Fidelity's but has an excellent long-term reputation, and the Vanguard custodial account is widely used.

Either is a fine choice. Most new investors find Fidelity's interface simpler.

Step-by-step: opening a Fidelity custodial account

1. Go to fidelity.com and click "Open an Account"
2. Choose "Custodial Account" from the account type menu
3. Enter the custodian's information (your name, SSN, date of birth, address)
4. Enter the child's information (name, SSN, date of birth) — you will need their Social Security number
5. Link a bank account for funding and transfer your initial deposit (no minimum)
6. Once funded, search "FZROX" and select "Buy" — you can invest in dollar amounts, no need to buy whole shares

That's it. Total time: 15–20 minutes. The account will show the child's name and your name as custodian. You manage it until they reach the age of majority in your state.

What to buy: the case for one index fund

For a child's long-term investment account, the best first investment is almost always a total U.S. stock market index fund. This gives exposure to thousands of companies across every sector — the same fund that Warren Buffett famously recommended ordinary investors use.

• At Fidelity: FZROX (Fidelity ZERO Total Market Index Fund) — 0.00% expense ratio
• At Vanguard: VTI (Vanguard Total Stock Market ETF) — 0.03% expense ratio, or VTSAX if you have $3,000

The expense ratio matters over decades. A 0.5% fee on a $100,000 balance costs $500/year in lost compounding. A 0.03% fee costs $30. At 18 years, this difference is meaningful.

A real example: parent opens a Fidelity custodial account with $1,000 and buys FZROX. At 7% average annual return, that $1,000 becomes approximately $3,380 in 18 years — without any additional contributions. Add $100/month for 18 years and the total is approximately $48,000. See the full compound growth math here.

Tax implications: the kiddie tax

Investment income in a custodial account is taxed — this is different from a Roth IRA or 529. Here's how it works for 2026:

• For children under 19 (or full-time students under 24), the first ~$1,350 of unearned income (interest, dividends, capital gains) is tax-free
• The next ~$1,350 is taxed at the child's rate (usually 10%)
• Unearned income above ~$2,700 is taxed at the parent's marginal rate — this is the "kiddie tax"

For most families with modest custodial accounts, the kiddie tax is not a major concern. A $30,000 account growing at 7% generates about $2,100 in gains per year — right at the threshold. For larger accounts, consider the comparison table below.

What happens when the child turns 18 (or 21)?

At the state-specified age of majority, the custodian's control ends and the account belongs to the child outright. There is no mechanism to take it back, restrict how they use it, or delay the transfer. This is the core tradeoff of a UTMA/UGMA account: flexibility in what the money can be used for, at the cost of parental control at 18.

If you're concerned about an 18-year-old receiving a large sum unconditionally, this is a reason to also consider a 529 (restricted to education) or to have ongoing conversations with your child about the money as they approach adulthood — so the transfer is expected and discussed, not a surprise.

Comparison: which account for which goal?

Many families use a combination: a UTMA for general investing (birthday money, grandparent gifts), a 529 for college savings, and a custodial Roth IRA once the child has earned income. See The Custodial Roth IRA for why the Roth option is so powerful for working teenagers, and 529 Plans Explained for the education savings angle.

How much to contribute: a realistic savings plan

There's no minimum contribution required to keep a custodial account going, which means families can start small and build the habit before worrying about the dollar amount. What matters far more than the size of any single contribution is consistency over many years, because compounding does most of the work given enough time.

Here's what steady monthly contributions turn into after 18 years, assuming a 7% average annual return (a reasonable long-run assumption for a total U.S. stock market index fund, though actual returns will vary year to year and are never guaranteed):

The pattern holds at every contribution level: over an 18-year runway, roughly half of the final balance comes from money actually deposited, and roughly half comes from investment growth on top of it. That ratio shifts even further toward growth the earlier you start — a account funded from birth has years 15-18 doing dramatically more compounding work than years 1-4, simply because there's more money sitting in the market for longer.

Where the contributions actually come from

Most custodial accounts aren't funded entirely by parents writing a monthly check. In practice, the money tends to come from a mix of sources:

• Grandparent and relative gifts. Birthdays, holidays, and other occasions are a natural funding source — redirecting even half of the cash gifts a child receives into the custodial account, instead of spending it on toys, adds up fast over 18 years.
• A fixed "found money" rule. Some families deposit a fixed dollar amount into the account every time the child receives cash from any source — chores money, a lemonade stand, a first part-time job — as a simple, automatic habit rather than something requiring an active monthly decision.
• Annual lump sums. Rather than committing to a monthly transfer, some parents make one deposit per year, often around tax time or a birthday, funded from a bonus or tax refund.
• Automated monthly transfers. The most consistent approach for families who can commit to it: an automatic transfer from checking to the custodial account on the same day each month, treated like any other recurring bill.

The 2026 annual gift tax exclusion is $18,000 per giver, per recipient — meaning two parents and any grandparents can each give up to that amount per year to a single child's account without any gift tax reporting requirement. For virtually all families, this limit is far higher than what's actually being contributed, so it rarely becomes a practical constraint.

Common mistakes parents make with custodial accounts

• Leaving the money in cash. A custodial account opened but left sitting in the settlement fund (uninvested cash) earns little to nothing and misses the entire point of starting early. The account needs an actual trade placed — buying the index fund — after each deposit.
• Picking individual stocks instead of a broad index fund. It's tempting for a parent excited about investing to buy a single well-known company's stock for a child's account. A single stock carries far more risk than a diversified index fund, and picking winners consistently over 18 years is extraordinarily difficult even for professional investors.
• Not considering the kiddie tax before selling. Because gains above the kiddie tax threshold are taxed at the parent's marginal rate, a large one-time sale (for example, to fund a specific expense) can generate an unexpectedly large tax bill in that year. Spreading a planned withdrawal across two calendar years, when possible, can reduce the tax impact.
• Overfunding relative to other goals. A UTMA account has no restrictions on how the child eventually uses it — but that also means it doesn't get the tax benefits of a 529 (education) or a custodial Roth IRA (retirement). Families should generally prioritize funding tax-advantaged accounts to the extent they're eligible before building a very large flexible UTMA balance.
• Forgetting to actually talk to the child about the account. An 18-year-old who discovers a six-figure account they didn't know existed is in a very different position than one who has grown up understanding the account, its purpose, and a plan for how it will be used. The financial education matters as much as the balance itself.

International diversification: should the fund be U.S.-only?

FZROX, VTI, and VTSAX are all U.S. total market funds — they don't include international stocks. For a first account, this is a reasonable and simple starting choice, since the U.S. market represents a large share of global market capitalization and U.S. total market funds are widely available with rock-bottom fees.

As a custodial account grows larger, some families choose to add a small allocation to an international index fund (for example, Fidelity's FZILX or Vanguard's VXUS) alongside the U.S. fund, to diversify beyond a single country's stock market. A common simple starting split for families who want this is 80% U.S. total market, 20% international total market — though there's no universally "correct" ratio, and a 100% U.S. total market fund remains a perfectly reasonable choice, especially for a first account or a smaller balance where simplicity matters more than fine-tuning the allocation.

Real example: two families, two strategies

Consider two families who each open a Fidelity custodial account when their child is born. The Ahmed family commits to $150/month, automated, no exceptions, for 18 years — total contributions of $32,400. The Chen family doesn't set up automatic transfers, but deposits whatever cash gifts the child receives at birthdays and holidays, averaging out to roughly $600/year, or $10,800 over 18 years.

At a 7% average annual return, the Ahmed family's account grows to approximately $64,000 by the child's 18th birthday. The Chen family's account, funded at roughly a third of the Ahmed family's contribution rate, grows to approximately $21,000. Neither approach is wrong — the Chen family's approach costs nothing extra out of pocket beyond redirecting gifts that would otherwise go toward the child's spending money, while the Ahmed family's approach requires an ongoing budget commitment but produces a meaningfully larger outcome. The right choice depends entirely on what a given family's finances can sustainably support — an automated $50/month commitment that continues uninterrupted for 18 years will, in nearly every case, outperform a larger contribution that gets started with enthusiasm and then abandoned after two years.

Dollar-cost averaging vs. lump-sum investing

When a family receives a larger sum at once — a $5,000 gift from a grandparent, for instance — a natural question is whether to invest it all immediately or spread the purchase out over several months. This is the same lump-sum vs. dollar-cost-averaging decision adult investors face, and the research on it is fairly consistent: historically, investing a lump sum immediately has outperformed spreading it out roughly two-thirds of the time, because markets rise more often than they fall over any given period, and money sitting in cash while being gradually invested misses out on time in the market.

That said, dollar-cost averaging (investing, say, one-third of the amount per month over three months) has real psychological value for a nervous first-time investor, even if it's not the mathematically optimal choice in most historical periods. For a custodial account with an 18-year horizon and no near-term need for the money, the difference between the two approaches is a rounding error over the long run. Parents shouldn't let the decision become a source of stress — either approach, chosen and then left alone, works.

Fractional share investing has made this decision easier than it used to be. Both Fidelity and Vanguard support buying index funds in exact dollar amounts rather than requiring a purchase of a whole number of shares. A $47 birthday check can be invested in full, immediately, without leaving a leftover $12 sitting uninvested in cash because it wasn't enough for a whole share.

What happens if the child doesn't want to keep investing at 18?

Some parents worry about handing over a large, invested account to an 18-year-old who might immediately liquidate it to buy a car or take a trip. It's worth being clear-eyed about this: once the custodianship ends, there is genuinely no legal mechanism to prevent that. This is the tradeoff inherent to UTMA/UGMA accounts, and it's the main reason some families choose to also use a 529 (locked to education use, with a tax penalty for anything else) or simply have years of ongoing conversation with the child about what the money represents and why it was set aside.

In practice, many 18-year-olds who have grown up watching a specific account grow — and who understand its purpose — choose to keep it invested, use it toward a graduation milestone like a laptop or a first car down payment, or fold it into a first home down payment years later. The single biggest predictor of what a young adult does with the account isn't the account structure itself, but whether they were involved in understanding it along the way. A teenager who has watched statements, understood the index fund concept, and maybe even helped pick a contribution amount is meaningfully more likely to treat the account as long-term wealth than one who is handed a number they've never seen before.

A practical middle ground some families use in the final few years before the transfer: sitting down together once a year starting around age 14 or 15 to review the account balance, talk through what index funds are and why fees matter, and let the teenager weigh in on decisions like whether to add an international fund. By the time control legally transfers, the account isn't a surprise — it's something the young adult already understands and, in many cases, has already started to feel ownership over.

Frequently overlooked practical details

• Dividends get reinvested, not paid out. Both FZROX and VTI/VTSAX pay quarterly dividends. Most brokerages default to automatically reinvesting these into more shares of the same fund, which is almost always the right setting for a long-term custodial account — check that this setting is enabled when the account is first opened.
• The account needs an EIN or the child's SSN, not the parent's. Even though the parent is the custodian and manages all trading decisions, the account is legally titled to the child, and the child's Social Security number is used for tax reporting (via the parent's return, if income is below the kiddie tax threshold, using IRS Form 8814, or via a separate return for the child above it).
• Multiple children need multiple, separate accounts. A custodial account can only have one beneficiary. Parents with more than one child need to open a separate UTMA for each, which also makes it easier to keep each child's contributions and growth clearly separated rather than needing to split a shared account later.
• State of residence determines the age of majority. This detail is easy to overlook when opening the account online, since the brokerage's default may not match the family's state. Confirming the applicable age (commonly 18, 21, or in some states as late as 25 depending on account type) avoids a surprise later.
• Moving the account if you relocate to a different state. The custodial account itself doesn't need to be closed or reopened when a family moves, but the age-of-majority rules that apply are generally determined by the state the account was established under, not necessarily the family's current state — worth confirming directly with the brokerage if this situation applies.