If multiple devices are used in your household, make sure the restrictions are enabled on all of them.
If you have a child between the ages of 13 and 18, you might want to try Family Sharing. Family Sharing makes it easy for up to six people in your family to share iTunes, iBooks, and App Store purchases. Family Sharing also has an Ask to Buy feature where an adult must approve a purchase before a child can buy an item. For more information, see these pages: