How to add a child to screen time

Set up Family Sharing - Apple Support

Invite people to join your family

You can be a part of only one family at a time.

On your iPhone, iPad, or iPod touch

1. Go to Settings.
2. Tap your name, then tap Family Sharing.
3. Tap Add Member.
4. If your child doesn't have an Apple ID, tap Create an Account for a Child. But if your child already has an Apple ID, tap Invite People. They can enter their Apple ID password on your device to accept the invitation.
5. Follow the onscreen instructions.

If your child is registered as an adult you will have to follow these instructions to change their birthday from your child's account.

Update the birth date associated with your Apple ID - Apple Support