If your contract states they don't pay on days your close for illness then they shouldn't pay on a day you were closed for illness.

If I were the parent and found out you didn't charge the other families but charged me because I had given notice that I wasn't planning on sending my child that day then my response would be to not tell you ahead of time that I didn't plan to send my child, as I was penalized for being diligent and respectful!