To control the levels of security for each step of the purchasing process, go to your device's Settings > General > Restrictions.
In the Restrictions menu, you're able to enable or disable your restrictions. If you wish to enable them, the system will require you to enter your passcode.
After the restrictions are enabled, scroll down to "In-App Purchases". Here, you can choose to completely disable every in-app purchase (turn the settings to OFF). Alternatively, you can manage the frequency of the password request using the "Require Password" button, as seen below:
If you want to keep the ability to make in-app purchases, you can set the device to require your password every time you make a purchase (select "Immediately") or every fifteen minutes.