For a kid, I think a total stock market fund has several benefits over a target date fund. Explaining a target date fund would be more complicated (why are they paying foreign taxes on a total international fund? what's a foreign tax credit?). Explaining that they own "a small part of every business in America" is easier. Since the custodial account is presumably in taxable, that also makes taxes simpler: it's just stock dividends (mostly qualified).
When there's a stock market correction, you might have a big teachable moment on your hands. You might want to brace them for that...