My contract states that if I am open then parents pay full fee. I use analogies with parents, such as if you go on holidays and you have a mortgage do you tell the bank to give you a discount for the time you are not living in the house, or the same with a car loan do you tell the bank you are not going to pay the loan because you are on holidays and not using the car. I don't think so. Why should my service be any different?
I have also mentioned to the parents that I will not charge them if they can find someone to fill the space while they are away. The family they send to me has to fit into my daycare, if not then they have to pay.
This is your business and they need to follow the contract otherwise they will try to get a "deal" a lot more times.

Good luck!!