Experience has been heard on Twitter that a little boy paid hundreds of thousands of yen for a game app. Fortunately, the refund seems to have worked, but in a family with children, it's not someone else. Parents are responsible for lending the smartphone to the child, even if a screen requesting approval is displayed, and it is quite possible that the child will not be refunded.
In that case, measures are needed. In-app billing itself is not a bad thing, but it's safer to make sure you can't pay without parental consent each time. In addition to billing, you also need to be prepared to install new apps randomly.
Up to iOS 11, the setting of "General" → "Function Restriction" of the "Settings" app was provided with a switch, but from iOS 12, the "Screen Time" of the "Settings" app → "Restrictions on content and privacy" → "Purchase on iTunes and App Store".
On the "Purchase from iTunes and App Store" screen, you can select permission / rejection for three items: "Install", "Delete App", and "In-App Purchase". By default, all permissions are selected, so let's change "install" and "in-app purchase" to non-permission.
When the change is completed, return to the previous screen (screen time), tap "Use screen time passcode" and register the passcode. If you do not set a passcode, you may change the settings on the "Purchase on iTunes and App Store" screen.
Easy explanation of operation procedure