Roblox is draining my bank account

Apparently my 9yo has been using my linked credit card to buy virtual currency for Roblox games. Noticed a bunch of 99 cent charges that added up to like $80 last month. I thought I had parental controls set up but clearly missed something. He says all his friends are buying stuff so everyone had some skins or whatever. How do you guys handle in-game purchases with your kids?