Compound interest fund as a kids' piggy bank: does it really work?

Compound interest fund as a kids' piggy bank: multiplying savings or a tax mirage?

Investing 5,000 euros at a child's birth in an index fund tracking the S&P 500 and waiting 30 years sounds like fairy-tale math: at 6% a year it triples in two decades, to around 16,000 euros. But the debate isn't just about returns — it's about who controls that money, how it's taxed, and whether the tax gymnastics are worth it.

The numbers: how much does it really grow?

A single 5,000-euro contribution at 6% a year over 20 years returns around 16,000 euros, according to calculations shared by readers. Add regular contributions and the figure skyrockets. However, index funds have recently delivered returns of around 20% a year, which invites optimism… or the statistical trap. The past doesn't guarantee the future, and concentration in the S&P 500 adds geopolitical risk: the U.S. is not invulnerable, and betting only on that index is a bet on dollar hegemony and military power, as some analyses point out.

The tax and control dilemma

In Spain, index funds are only taxed when sold, with a 1,600-euro annual capital gains threshold for filing. But withholding tax is applied automatically upon redemption. Ideas come up like moving to Andorra to pay less tax, though the logistics (local bank account, effective residency) are complex. More relevant is the debate over who manages the money: junior accounts transfer control to the child at 18, risking that a teenager squanders the capital. That's why many prefer to hold the fund in their own name and help their child when needed, rather than handing over a lump sum unsupervised.

What if the S&P 500 isn't the best option?

Some investors distrust the U.S. index and opt for the MSCI World or seek exposure to emerging markets like China and India. The argument: America's glorious past doesn't guarantee its future. Alternatively, you can switch funds without paying taxes until the final redemption. As always, the decision is a balance between expected returns and risk tolerance.

The debate leaves an open question: is it better to have a piggy bank that teaches financial literacy, or an investment fund that could vanish with a reckless teenager? The answer isn't in the numbers, but in how the family plan is designed.