that's a nice and smart move from you, props to you.
Saving something every months (fiat, btc) for our childrens it's the best way to give them something to start with, you never know what's the future holds and if now we are able to save something that's something that everyone should do, think about our childrens!
I would buy an hardware wallet in a security box in a bank (or hidden somewhere safe aka fireproof, waterproof, dustproof, etc.etc.) with the instructions and give all the details to another trusted person, that's it.
That's a very good idea! Even though I have no kids, I am planning to do things to my parents. That's a very wise thing to do and thank you for your idea. It will be very helpful for us.