If you want to avoid accidental in-app purchases, you might want to adjust your settings in the Google Play Store.
Please follow these steps:
- Access the Google Play Store
- Go to "Settings"
- Go to "User Controls", then activate the setting "Use password to restrict purchases"
- Enter your new password and choose confirm.
- Any in-game purchases after having activated these settings will first require you to enter your security password.
In addition, if someone else is using it on a regular basis, you might want to create a different profile for them. Go to: Settings --> Users --> Add user, then follow the on-screen instructions.