Another possibility would be for the deposit to show up in the passbook like an authorized deposit, but with some visual indication (gray text?) that the deposit is being held.

Since the kid treats a deposit as a deposit, regardless of whether or not it's authorized, the passbook should display both values.

One solution is for the deposit slip to be shown to any kids who look at their passbooks, but the deposit won't show up in the passbook proper until the deposit slip is authorized by a parent.