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.
You should absolutely not be trusting your memory, especially not over a period of 20 years. You can guarantee at no point in the next 20 years you will be knocked on the head, be in a car accident, have a stroke, or any of the other 100 things that can cause memory loss? I don't think so.
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.
This is equally bad advice. You should never "pick" a seed phrase. Humans are bad at being random, and anything you pick is therefore weak. Seed phrases should be generated by your wallet from your random seed number.