Why don't you just tell the kid when he grows up the 12/24 word mnemonics? Depending on how much money we are dealing here you can probably just remember the words. Not that difficult.

When you make the wallet, just pick a mnemonic where you can piece together a poem or story and just tell it to the kid its entire life. When they grow up, tell them what the poem or story really meant.

Sure there are issues here where you can forget the words or just accidentally pass away but there are also issues with paper wallets or steel engraved bars which can also be damaged or stolen or misplaced after 20 years.

The USB is not a good method because who knows, maybe the chip inside the USB can degrade like some of those old burnt CDs from the 90s and just lose its data.