The full time child should pay for all days they are scheduled, regardless if family is in town or not.
Before/after needs to be more clear. Are you open to them for full days on Xmas/march break/PA days etc. If so, then you should apply the full day rate for preschoolers if they attend all day during the breaks. I used to do before/after, and they paid for their regularly scheduled days during all breaks($25 before/after rate) if they did not attend, and if they came in, they paid a full day preschool rate($45 a day) You are holding their spot open, so they should still pay something.