I've only closed once this year - unfortunately, it was for a full week - my youngest had the "dreaded" H1N1. I told the parents they didn't have to pay, but one did anyway. It was so nice of her and I have given her a couple of breaks since then.
My policy states that if I close for my own illness or for my kids' illnesses, I don't charge. However, the parents are paying for the spot and not the days, so if I'm open, then they pay if they choose to stay home.