Most apps that have in-app purchasing list "Top In-App Purchases" on the left-hand sidebar on the App Store listing.
If you go into Settings > General > Restrictions, you can set restrictions on the use.
* You can set a different PIN passcode
Optional restrictions available:
* installing and deleting apps
* set age limits on apps
* restrict in-app purchases
You need to enter your Apple ID to make purchases but it's worth deciding on whether to give a 15 minute window for purchases without re-authentication or to require immediate re-authentication.
If the kids know the Apple ID passwords and passcode, none of that will provide much protection.
If you go into Settings > General > Restrictions, you can set restrictions on the use.
* You can set a different PIN passcode
Optional restrictions available:
* installing and deleting apps
* set age limits on apps
* restrict in-app purchases
You need to enter your Apple ID to make purchases but it's worth deciding on whether to give a 15 minute window for purchases without re-authentication or to require immediate re-authentication.
If the kids know the Apple ID passwords and passcode, none of that will provide much protection.