Before letting a child or teenager loose with an iPhone or iPad, you may decide that you don't want them to be able to purchase content without your consent. If blocking in-app purchases isn't enough, you can completely limit their access to the App Store, iTunes, and iBooks through restrictions. This way they can't purchase any content without asking.
How to block access to iTunes, App Store, and iBooks on iPhone and iPad
Launch the Settings app on your iPhone or iPad.
Tap on General.
Tap on Restrictions.
Tap on Enable Restrictions.
Choose a password that no one else will know and confirm it.
Under the Allow section, turn Off the options for iTunes Store, iBooks Store, and Installing Apps.
You'll notice when you return to the Home screen that the icons for all three apps are now hidden from sight and can't be accessed at all. It's now safe to hand the iPhone or iPad over. If and when you'd like to make them re-appear in order to purchase an app, song, or book, just enable them the same way you disabled them temporarily. Then before you hand back over the iPhone or iPad, just enable them again.