While this doesn't apply to me what I would do if it was me is have the families pay for your vacation based on weeks of care not days of care. So if you take a week off, they pay for the days they would have attended during that week. If you only take a few days off, again they pay for the days they would have attended.
When you get paid vacation you are just trying to get compensated for the money you would have made during those days...you are not trying to profit more (having the family pay for all the days you take off even if they would not have paid for care if you were open).
The same would be reflected if you allow families to take vacation time (unpaid). If you allow full time children 2 weeks off unpaid vacation (theirs, not yours) then the part time children would not get 10 days , they would get the same 2 week period (how ever many days they would have attended whether it be 4 or 6 or what ever).
The most important thing here is that your contract reflects your plan and that you ensure the families are aware of this when they sign (and I would direct them back to their contract before you take time off to remind them the first time).