I think this would depend on the families. Do the kids need before and after school care on the days they are in school? If so, they would need to pay you full time and use you full days on their off days and before/after on their school days.

If you have 2 families that only need care on their off days they could share a spot. If you don't have 2 kids to share a spot I would lean charging for the full time spot even though they won't use it full time. But, this would really depend on what works in your area, how many other spaces are available in your area and what your demand is.

My hometown in Ontario has been doing it this way for many years. Junior Kindergarten is 2 full days a week and senior kindergarten is 3 full days a week. It took a good while for the local daycares to get a system down pat to accommodate it.