Don't worry about the charges, the card will be cancelled immediately!!! My DH lost his a couple of years ago and we went to guest services in the park where we were at and they handled it for us. They immediately cancelled his card and then issued us a new one. The great thing of having the charging privileges on your room key is that the charges don't go over to your credit card company until you hit the limit so if you do lose the card nothing has actually been submitted to the credit card company yet anyway so your safe.
I don't know if your travelling with children but just be sure to put the charging privileges on the adults keys only.