I would offer 2 contracted spots.

One spot is for the off days in the regular off kinder days in the school year. This spot does NOT provide any care on school holidays. A family with grandparents nearby can fill this.

The other spot is for the regular off kinder days in the school year (the opposite days of the above spot) PLUS the school holiday days. This spot MUST pay for very school holiday whether or not care is used.

The 2 spots will fill one full time position with no overlap. Just make sure your contracted terms are very very clear and each family knows at signing what they get and what they pay.