In app purchases

Apple offers several suggestions for restricting in-app purchases in the support document Prevent in-app purchases from the App Store:

You can control purchases on an Apple device in a few ways:

• If children in your Family Sharing group have their own devices, use Ask to Buy to screen and approve their purchase requests.
• If you let children use your device, you can require a password for every purchase.
• You can also turn off in-app purchases entirely using Screen Time on your own device or a child's device.

There are further details on that page on how to use Screen Time to prevent in-app purchases.

If you don't use Family Sharing read What is Family Sharing?