I don’t know if any Apple engineers are around here, but an option for restrictions in iOS where you can block applications from appearing in the App Store that

I don’t know if any Apple engineers are around here, but an option for restrictions in iOS where you can block applications from appearing in the App Store that include in-app purchases would be a good solution to this problem. Then even if you let your kid download apps on their own they can’t fall into this trap. And if they were in an app and accidentally clicked on an ad for another app it wouldn’t resolve. (Family Sharing and Ask to Buy are supposed to fix this by sending the request to the parent’s Apple ID, but I don’t know anyone who uses that feature with young kids. And it’s not easy to set up, since the kid needs an email address AFAIK).