If you want them to have spending power, run LNDHub or LNBits and set them up with Blue wallet connected to it.
You still maintain custodial control.
If you want it on chain, then set them up with a wallet that is derived using BIP85. That way you have the seed words to restore and can help them move funds.
Or, consider a 2 of 3 multisig (You, Child, Parent)
