If you need to make the in-app purchase process more secure or want to disable them entirely follow the instructions below.
1. Open the Google Play Store app
2. Tap Menu > Settings.
3. Tap Require authentication for purchases.
4. Choose a setting.
5. Follow the on-screen instructions.
Source: Google Support
Apple
1. Go to Settings > Screen Time, then tap Turn on Screen Time. If you already had Screen Time turned on, skip to step 3.
2. Tap Continue, then choose 'This is My [device]' or 'This is My Child's [device]'.
• If you're the parent or guardian in your device and want to prevent another family member from changing your settings, tap Use Screen Time Passcode to create a passcode, then re-enter the passcode to confirm. In iOS 13.4 and later, after you confirm your passcode, you'll be asked to enter your Apple ID and password. This can be used to reset your Screen Time passcode if you forget it.
• If you're setting up Screen Time on your child's device, follow the prompts until you get to Parent Passcode and enter a passcode. Re-enter the passcode to confirm. In iOS 13.4 and later, after you confirm your passcode, you'll be asked to enter your Apple ID and password. This can be used to reset your Screen Time passcode if you forget it.
3. Tap Content & Privacy Restrictions. If asked, enter your passcode, then turn on Content & Privacy.
4. Tap iTunes & App Store Purchases.
5. Tap In-app Purchases and set to Don't Allow.
Source: Apple Support