This reminds me of Using Locktime for inheritance planning, backups or gifts: Coingifter could create a time locked transaction that's only valid when the kid turns 18. This way, he can receive all the information he needs to redeem the coins already, but can't use it yet.
Pros: Coingifter can forget about it.
Cons: The kid still has to trust Coingifter doesn't take the money earlier. And it requires more understanding of what you're doing because it's more complicate to create/redeem.