Use Screen Time to prevent purchasing on your iPhone, iPad, or iPod touch

To prevent in-app purchases, you need to turn on Screen Time:

1. Go to Settings > Screen Time, then tap Turn on Screen Time.
   
2. Tap Continue, then choose "This is My [device]" or "This is My Child's [device]."
   
   • If you're the parent or guardian of 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.
   • 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. 
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.
   

You can also go to Settings > Screen Time > Content & Privacy Restrictions, and tap Allowed Apps. Then deselect iTunes Store and Books.

Learn more about using Family Sharing with Screen Time. 

Make sure to choose a passcode that's different from the passcode you use to unlock your device. If you can't remember a restrictions passcode that you set on a previous version of iOS, or you forgot your Screen Time passcode, you need to erase your device, then set it up as a new device to remove the passcode. Restoring your device from a backup won't remove your passcode.

If you don't want to prevent in-app purchases or disable purchasing completely, you can require a password before downloading content from the iTunes & App Store.

If you use Family Sharing, you can let your child make purchases with Ask to Buy.

The iTunes Service is only available for individuals 13 and older or children under 13 who have received an Apple ID through the Apple ID for Students program. If your child is under the age of 13 and isn't an educational Apple ID holder, Apple must close and delete all information associated with their account.

With Screen Time, you can also control access to certain applications, features, and content types.