This has happened to me before. Child was sick the day before so paid as I was open but then next day I was sick and closed. I didn't charge any of the parents.
I try to treat these situations like how I'd want to be treated if I was in their shoes. I know it's hard to lose money but I'd be hurt if I was a parent and found out I was being charged when I shouldn't be, according to the contract. I'd feel taken advantage of and may even consider a new provider. I think the damage done by these kind of actions could hurt me in the long run more than just a days lost pay. If that makes sense...