Most people don't know this but the "Parent" (read:credit card holder) of a family sharing account can't see, let alone cancel, any subscriptions a minor on the account has purchased.
Let that sink in for a second. Apple will happily charge the parents' credit card a weekly recurring fee but there is nowhere in their interface (on device nor on the web) where the parent can even see that subscription.[0]
Apple expects the child to go into their interface and cancel the recurring subscription. Something many (most?) adults find confusing. In my case, the child just deleted the App when the trial was over. Which is of course perfectly logical thinking. No bueno.
So if the child is away at school (as in my case), or the phone gets left at a friend's house, or worse... stolen. The only way to get it cancelled is to call them.
There's more to this story, including details on how the built in parental controls are intentionally crippled (IMHO) but I've got to run. In summary: The entire "Family Sharing" system is built to rip off people while still maintaining plausible deniability.
[0] A senior Apple rep told me this is for privacy reasons. But two things: the minor should have no expectation of privacy when spending their parents money (how is that a good thing?). And secondly, Apple does email a receipt for the subscription to the card holder... so the privacy excuse was pure bullshit. In my case I missed the email because I have more than 6 or 7 recurring Apple subscriptions. I do take partial blame because of that. But I've still not been given a good reason why a parent can't easily cancel a childs subscription.