I agree. I charge for the spot and that includes sick days. I can't make a living if I don't charge every time someone stays home due to illness.

If you do the x amount of sick days per year, make sure you keep a record of it and have the parents initial every time they use a day so you don't run into "well I've only used 4 so far and have 1 left" when they really don't.

I think it's fine to change your policy now.