how do you track the amounts you set aside for them?

I'm guessing the simplest way, if the granularity of amounts you save for them is large enough relative to fees, would be to have one address per kid, and keep the addresses frozen so the wallet doesn't spend from them; then as they age into having their own wallets [or e.g. if they convince you to spend their sats for some request], you can drain all the UTXOs from that kid's address together, making the concerns about address reuse almost irrelevant.