how do i change my childs account from a adult to a child

On your iPhone, iPad or iPod touch

1. Go to Settings > [your name].
2. Tap Name, Phone Numbers, Email. You may need to enter your Apple ID password.
3. Tap Change Birthday or Birthday.
4. Update your date of birth, then tap Done. If you're changing the date of birth for a child, find out what to do.

Update the date of birth associated with your Apple ID