For me:-

Parent pay for stat if on their scheduled day. I not charge for the stat if it not their regular day.
Parent pay 100% fees if they don't attend for sick or vacation.
I have 10 paid day off and if I take more, anything over the 10 is unpaid.

The mean, parent pay same fees, every two week, regardless of attendance or if I am on vacation as long as I have not already had 10 days.