Is it in your contract that they have to pay regardless? You can't let them get away with it, they will likely take advantage as much as they can. They signed a contract they need to follow it. Tell them your children were away anyways and the other children attended, that it was theiur decision to keep their child home. Plus you told them ahead of time you were taking thursday off? She could have taken her son thursday to the doctor, not a day you were open.