This is the worst of being Grandma. It is so long since our children were the age of our grandchildren that we have forgotten the details of what it was like.
I bridged the gap teaching 9- 16 year olds, so I know that if you give a 9 year old a lengthy explanation of ANYTHING he closes his ears - thus disproving the zoologists who maintain that the only mammel capable of closing its ears is the seal!
I assume you are talking about a Visacard or something similar. If so, ring your bank ASAP and cancel the card and get a new one, after making sure you either have another credit card you can use in the interim, or enough cash to tide you over until the new one and the PIN turns up.
Mention to you grandson if you like that you have had to do this, as it seems he, or someone else, has bought games using your card, and that this has cost you a lot of money.
Leave it at that.
Did his parents know that you used your card and warned theechild not to continue doing so?
In your place I would feel that apologising to them might be called for - you have inadvertently led their child into doing something the adult world sees as dishonest.
If you talk to the child or his parents about this, you should make it very clear indeed that you did a foolish thing you now regret.