Roblox charges each kids when I buy for 1 kids on iPhone

Whenever I purchase Robux for one of my child’s iPhone I get charged for each child’s iPhone. It doesn’t show up immediately, it takes a few days, but it always does. How do I stop the multiple charges? I tried turning off family share but then I have to input a credit card for each kid when I tried to purchase for 1 child. I want parental control but not multiple charges.

This is a repeat question someone else asked but no replies.

anyone know how to fix this?