In my contract I charge all kids (full or part time) full rate on stats. When I had children before and after school I only charged them for the full days that they used. If they were away for the whole Christmas break they didn't pay but if they were supposed to come on a particular day they were charged for it. If they ended up changing the schedule last minute too bad they were still charged for it as they had already paid me.

In your case if it is agreed that the child is supposed to be with you on their Christmas break full-time then she should be paying full rate for the week. Make sure it is clearly stated in your contract though before you bring it up to mom. If you just let it be though be prepared for it to happen again because she will think it is ok and how things work