To protect yourself from accidental in-game purchases, you can enable authentication, after which making a purchase on the device will require you to enter certain information, such as a password.

To do this:
1. Open the Play Store app.
2. Tap on Menu, which is the three-bar button in the top left-hand corner of the screen.
3. Go to Settings.
4. Tap Require authentication for purchases.
5. Select one of the authorization modes.
6. Follow the instructions on the screen.