A blank SatsCard is probably the best way because it can be used to help educate your kids to send bitcoin to a new address and how to use it as a medium to long term savings.
You could also use something like a paper wallet or maybe a wallet that you also have the seed phrase for so funds could magically appear in the wallet and also be recovered if they ever lose their own seed phrase.
According to data gathered by the American dental insurance company Delta Dental, the average payout per tooth in the United States rose from $1.30 in 1998 to $6.23 in 2023
So the tooth fairy is now giving out about 21k sats per tooth.
LNBits - online wallet (under kinda your control, cuz that's still your kid :-))
I mean, practically tape a piece of paper to the dollar bill with link to lnbits :-) and stash under pillow.... When the worlds collide !!!