Turn on Ask to Buy on iPhone
When your Family Sharing group shares purchases, you can, as the family organizer, require that young family members request approval for purchases or free downloads.
Go to Settings > [your name] > Family Sharing.
Tap the name of the person who needs to request approval, then turn on Ask To Buy.
Note: Age restrictions for Ask to Buy vary by region. In the United States, the family organizer can turn on Ask to Buy for any family member under age 18; for children under age 13, it’s turned on by default.