I agree with the others, you shouldn't give any money back. They were in wrong and not following policy. They are likely just mad that you are standing up to them and not allowing their disrespectful attitude to win anymore.

I just recently had a child stay with gramma for 2 weeks and she asked if she could have my sick policy so she would know when/if she would need to keep her home. I appreciated that so much and made sure to tell her and the parents this!!