I charge half fees during holidays for under fives as I only have 3 under 5 spaces and can't afford to tie them up with not being paid in the hols. Because they pay half fees they can use the place in the holidays if they want to - and all of them do from time to time (paying the other half of the fees for the time used).
I have a number of older children age 7 and older who come p/t in termtime - they pay session rates and nothing in the holidays because their place is only available in term-time. I have different children after school than before school, so if they all came in holidays there would be too many.