Honestly if you haven't discussed this with the parents before now I think you have a serious issue if the parents are only expecting to pay the 20 its a little late to be telling them they have to pay more now. What happens on the PD days? Are you taking the next week off and still charging them it kinda looks like that is what you are saying?. I have a set monthly fee parents pay me for before and after this includes all PD days one week at Christmas and one week in the Spring. If they pay 35 for PD days then they are most likely expecting to pay that so it shouldn't be a problem