Hello enes hester,
I'm sorry to hear that happened. A couple of things to sort through here, since "cancel the card" and "refund the charge" are actually different.
Cancelling or replacing the card itself is something only your bank or card issuer can do — Microsoft doesn't have any control over that side. Give them a call using the number on the back of the card, tell them the charge was unauthorized, and they can block the card, issue a new one, and start a dispute on the specific transaction.
For the Robux charge itself, the refund path depends on where it was bought:
- If the charge on your statement shows as Microsoft or Xbox, you can request a refund at https://support.xbox.com/help/subscriptions-billing/buy-games-apps/refund-orders, signing in with the Microsoft account that made the purchase.
- If it shows as Roblox directly, then Microsoft can't refund it — you'd need to go through Roblox Support at https://www.roblox.com/support, since the charge went straight to them.
(Disclaimer: This is a non-Microsoft website. The page appears to be providing accurate and safe information. Watch out for ads on the site that may advertise products frequently classified as PUP (Potentially Unwanted Products). Thoroughly research any product advertised on the site before you decide to download and install it.)
To stop this from happening again, the most reliable fix is setting up your granddaughter's account under Microsoft Family Safety so any purchase she tries needs your approval first. You can do that at https://family.microsoft.com. It's also worth removing any saved card from any Microsoft account she has access to.
I hope this helps clarify things.