How can I prevent accidental purchases in the game on Android?

On Android devices:

1. Open the Google Play Store app.
2. Tap the menu icon and select Settings.
3. Tap Require authentication for purchases.
4. Choose one of the authentication options.

●       For all purchases through Google Play on this device. Authentication is required for every content purchase through Google Play (including within apps).
●       Every 30 minutes. Every time you authenticate for a purchase, you can continue to buy all forms of content through Google Play for the next 30 minutes without authenticating again. If you enter your password once, then all subsequent purchases within the next 30 minutes will be completed without requesting your password again.
●       Never. If you choose this option, authentication will not be requested to complete a purchase. As a result, anyone with access to your mobile device will be able to make purchases. You assume responsibility for all charges for any purchases on your account.

Note. Authentication is always required for every purchase under the Family section for an app or game, even if you have your settings set differently.

Follow the instructions on the screen.
You can find more detailed information about protecting your password and purchase authentication methods on this Google Play Help page.