How to restrict in-app purchases

Apple:

If your device is regularly used by someone other than yourself, it is a good idea to restrict in-app purchases within the app. You can disable in-app purchases using the below instructions:

Step 1:
You can set up authentication tools for your Apple account that mean a password is required when making an in-app purchase (we would probably advise against using TouchID or FaceID). It's also worth making sure that the option for your password to be required on all in-app purchases is also enabled.
For further information: https://support.apple.com/en-gb/HT204030

Step 2:
You can completely disable in-app purchases using Screen Time.
For further information: https://support.apple.com/en-gb/HT204396

Android:

To disable in-app purchases, please follow the below steps to prevent purchases being made on your device without your permission:

• Open Google Play
• Tap the Menu icon and then select “Settings”
• Tap “Require authentication for purchases”
• Select “For all purchases through Google Play on this device”
• Enter your password when requested

There are three purchase authentication options available in the Play Store app:

For all purchases: Authentication is required for every digital content purchase through Google Play (including within apps) on your device.

Every 30 minutes: Every time you authenticate for a purchase, you can continue to buy all forms of digital content through Google Play (including within apps) for the next 30 minutes without authenticating again.

Never: Disabling authentication may result in unauthorised purchases. You assume responsibility for all charges.