How to restrict in-app billing
In order to prevent unintentional in-app purchases due to misoperation etc.,
you can add password restrictions in GooglePlay store settings.
1:Launch the Play Store and tap the menu button on the top left.
2:From the menu, tap "Settings".
3:Tap "Require authentication for purchases".
4:Select "For all purchases through Google Play on this device"
5:As the password entry screen is displayed, enter the password and tap "OK".
The password is the one at the terminal setting (same as the password used in Gmail etc).
By doing the above setting, you will need a password for in-app billing and you can prevent unintended billing.
For details, please see Google official Require a password or authentication for purchases