Like every other investment such as real estate, stocks and other forms of investment we know that you can invest in that kind of things and then you get something like say a Certificate or a Binding Document as a proof that you are part of that investment, now that of bitcoin all you have is just your address, your password, key and seeds as proof of the ownership of the wallet.
You can have shared addresses which would be used by two people something like an joint account.On;y compulsion is the other party should be very trusted.Otherwise they can use multi-escrow system.
BTCSimple,create a dedicated wallet for your child,paper wallet the private keys and never look back at it again until the day you're handing it over to your kid.There are multiple ways to do this and it's subjective.