If all you're doing with a bunch of cash over 18 years (!) is holding it in a bank account to collect interest, you're just losing money to inflation. Bank interest is almost nothing. Better to hold stocks, or at the very least bonds if you're extremely risk-averse.

Though this needs to be put in context - maybe you actually intend for the child to be permitted to spend the cash, in which case a bank account makes plenty of sense.