How to add your child onto Family SharingHow to I add a child account onto Family Sharing? Like this!
You can add child accounts onto your Family Sharing setup for any child under 13. That way you can prevent them from buying things willy-nilly on your dime. Creating a child Apple ID creates an "Ask to Buy" button, which notifies you when your kid wants to purchase an app, song, TV show, or anything else found on iTunes or the App Store (Mac App Store included). If your child already has an Apple ID, you can move them onto a child ID (sort of).
Here's how to add your child onto Family Sharing.
How to create a new child Apple ID
A child under 13 cannot create their own Apple ID. Adding a younger birthday halts the regular Apple ID creation process, so you have to do things a special way.
Launch Settings from the Home screen of your iPhone or iPad.
Tap your Apple ID at the top (should be the name you used to sign up).
Tap Family Sharing.
Tap Create an Apple ID for a Child at the bottom of the screen.
Tap Next on the top ...
Source:
How to add your child onto Family Sharing