Here's how you can restrict in-app purchases on your iPhone, iPad or iPod.
1. Go to Settings on your iOS device.
2. Go to General.
3. Tap on Restrictions.
4. Tap Enable Restrictions.
5. Type in a passcode.
6. Scroll down and toggle the "In-App Purchases" option to the Off setting.
For more, see the original article at the link below.