While Rumble Stars is free to play, you also have the option to purchase in game currency to progress faster.

To set restrictions on your iOS device, click here.

Alternatively, if you don't want full restrictions, set the device to require a password every time a purchase is made, or every 15 minutes: go to your device's settings -> App Store, iTunes Store-> Password Settings.

To set restrictions on your Android device, follow the instructions here.

It will explain how to turn authentication on and off, or at intervals of time.