Too bad. I guess I got the option there because notification started working for me. I only discovered that page after clicked on the the "Review the request button" in the messages would not work. (Nothing would happen on clicking it and sometimes the notification would not be fully expanded with the app info).
But based on your description I am not sure if you get to the screen I was mentioning. This was not the screen with messages, but rather a details of the family member. On this screen just tap on the avatar on top.
But again I might be wrong and you wont get anything there unless first the issue with notifications is resolved.
Maybe you can try what I did. I first on iPad clicked on a child in family, then in Ask to Buy option I disabled it.
For some reason it took like 5-10 minutes for the kids iphone to sync with that change. I even did a reboot of both ipad and iphone so not sure if it was the time waiting or the reboots.
Then I enabled it again... And after second time going through this cycle notification appeared.
Why and how I have no idea. As mentioned review option in messages did not work but after finding the family member details page (or whatever its called, I cant find the screen on the web) I noticed there info on the request and was abble to approve it from ther.
Good luck :)