Transfer of subscription and games to child's account

Nothing can be transferred between accounts. You can set the console as your Home console so he can access the games/subscription, but he would have to start over. https://support.xbox.com/en-US/help/hardware-network/console/my-home-xbox

I did the same thing. If I do create a new child account for him , can the Live subscription transfer to his account?

Adult accounts cannot be converted to child accounts. You would have to create a new account for him that is a child account, but keep in mind that nothing transfers between accounts. So he would have to start over on any of his games.

Would this also mean buying a whole new years subscription???

There is no way to transfer the account contents to another account and there is no way to make it a child account. He can still purchase things using redeemable currency codes and keep up the Xbox Live subscriptions with pre-paid subscriptions codes. But
in general it's always advised to create a new account for the child so it has all the correct info listed.

you can indeed transfer money from a parental account to a child account, can be done accidentally too, not sure if there is a way to transfer back as child account cannot transfer money.