Apple recently launched a new Families webpage featuring tips on how to keep kids safe on their iOS device. It covers everything from the safe browsing to Apple's "Ask to Buy" tool. Ask to Buy allows the master device of a Family Sharing account to approve or decline any app downloads. Here's how to set up Ask to Buy on your iOS device:
First you need to have Family Sharing set up on your device. Navigate to Settings then tap your name (On iOS 10.2 or earlier navigate to Settings > iCloud). Tap "Set Up Family Sharing," then tap Get Started. Follow the instructions then invite the family members who want to join. You can add up to 6 people to a Family Sharing account.
Once Family Sharing is set up you can then set up Ask to Buy. Go to Settings. then tap your name, then tap Family Sharing. From here you can select the members of your family who you want to approve app store purchases. Tap Ask to Buy, and you're done. The selected person will now need your approval for any App Store downloads.