I had a similar misunderstanding with a parent a couple of weeks ago. They didn't think they needed to pay for stat holidays -- 8 other families didn't have any issues with the wording of the contract, so I knew it wasn't that hard to figure out. But I thanked them for bringing the "possible ambiguity" of the wording to my attention, and let them know I'd change it to make it more clear. But I still charged them for the stat in question
. Just be polite, but stick to your contract. If you let her off the hook this time, she may think you will next time too!