How do I restrict in-app purchases on my Android device?

Google has a good few ways of ensuring that your device is protected against unintended purchases, and the best way is requiring a password before any purchase can be made.

Open the Google Play Store on your device.
Tap the Menu (the three lines) on the left side of the search bar.
Scroll down to Settings.
Select the Require Authentication For Purchases option.
Ensure the first option, indicating all purchases, is selected.
When setting this up or disabling it, Google will usually ask for your account password.

Automatic In-App Purchases are now disabled and a password will be required to enable purchases on your device.

How to use your fingerprint scanner for purchase authentication

Open up the Google Play Store on your device.
Tap the Menu (the three lines) on the left side of the search bar.
Scroll down to Settings.
Select the Require Authentication For Purchases option.
You need to make sure you select the first option. That’s how you apply the setting to all purchases.
When setting this up or disabling it, Google will usually ask for your account password.