Apple support provides a helpful article for limiting purchases on your child’s phone or iPad. It provides clear instructions for preventing unauthorized purchases from your iTunes Account (apps, music, movies, and books) using the “Restrictions” feature in Settings. Warning: Once you the required Restrictions passcode, there is no back door. Forget it, and the device will have to be erased resulting in data loss (if you try to restore from an iTunes/iCloud backup the Restrictions passcode is restored as well!)
With In-App purchases restricted, the following will appear when “Continue” is selected to complete purchase:
Another option for limiting purchases is through “Ask to Buy” a feature of Family Sharing.