Put it into a Roth IRA. That way it compounds tax-free in the worst case that they do pull it out at some point.

While the regular contributions can be pulled out at any time without tax or penalty, certain rules have to be followed to be able to pull out the earnings without tax or penalty.

Which -- if they use it for home purchase ...

There are certain rules about pulling out money for a "first time home purchase" (if one were not yet 59.5 years old and had a Roth IRA for at least 5 years), but that exempts the earnings from penalty, but one would pay income taxes on the earnings.