Ask Parent/Guardian feature
There appears to be a bug with this. whenever my children select to ask a a parent, we never get the notification.
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On your iPhone, iPad, or iPod touch
Tap Family. If you don't see Family, tap your name, then tap Family Sharing. If you're the family organizer, make sure that Purchase Sharing is turned on. Then you can tap a family member's name to see if Ask to Buy is turned on for their account.