iOS

To set in-app purchase restrictions:

1. Open the Settings app and go to Screen Time. Enable Screen Time if you haven’t already done so, then tap Content & Privacy Restrictions.
2. Tap the switch next to Content & Privacy Restrictions to enable the options below.
3. Tap iTunes & App Store Purchases.

On Apple devices, you have two options to restrict in-app purchases: always require a password for in-app purchases or disallow in-app purchases entirely.

To disable in-app purchases entirely, tap In-app Purchases and select Don’t Allow. To require a password for in-app purchases, select Always Require under REQUIRE PASSWORD.

Visit Apple Support for more information on enabling purchasing restrictions. Parents can also use the Ask to Buy feature to limit their children’s access to purchasing.

Android

On Android, you can restrict in-app purchases by requiring a password for each in-app purchase:

1. Open the Google Play Store app.
2. Tap Menu > Settings > Require Authentication For Purchases.
3. Select the ""For all purchases through Google Play on this device"" option.

For fingerprint authentication: 
Go to Settings and check the box next to Fingerprint authentication. 
Enter your password. 
You'll only need to enter your password when you first set up fingerprint authentication.