My Family is the way to turn on and manage app and game downloads for your child's Windows Phone. When you set up a "family" (which includes your child's account and one or more parent accounts) you'll be able to choose what types of app downloads you'd like to allow (free, paid, both or none). You'll also be able to set the rating level for the games your child can install.
To see a Windows Phone on the My Family page, you'll need to first sign in to the phone with your Microsoft account. Then, when you sign in to My Family with the same account, the phone will show up.
If you've done this and you still don't see your phone on the My Family page, the phone probably needs a little time to sync.
Source