I've been giving my nieces and nephews a small amount of Bitcoin every year. I kept it simple (and insecure) and made paper wallets for them and told their parents to keep them in a safe.

I currently have a copy of the private keys. When the time comes, I'll sweep them to hardware wallets for which I have no keys nor PINs.

It's pretty fun every Christmas. "Hey Vinney, I heard Bitcoin is up". "Yea, remember that $n Bitcoin I gave your kids so far? Now it's worth $n*x. Keep it safe!"