Agree that the issue is with Roblox and not with Apple.
It would be like challenging a phone bill with O2, and them saying go to Apple.
You have probably reached the end of the appeal process with the customer services people, and they may have a limit on the amount they can refund without going higher?
It's definitely a PR issue for them, but when I mentioned this to DS (12) he said that Roblox have a reputation for having an in app purchasing system that isn't entirely clear, so no wonder your 8 year old found it easy to purchase so many times.
The other idea is to check an Ombudsman Code of Conduct? Would it be gaming/gambling? Is there anything in the Code of Conduct to appeal a decision? You would need to have an email trail.
Personally I think that in lockdown, so many parents are happy to give their children a tablet to keep them occupied, and unless you were standing over them, you wouldn't realise they were making in app purchases, so the once a month statement seems a bit late.
I definitely won't be subscribing to Roblox.