I always find it interesting why people have this idea that summer camps are more expensive. I think it is relative to the area you are in but not true of the whole by any means. Home Daycare in the GTA is far more expensive than summer camps. I live in Niagara Region where income is the lowest of the whole golden horseshoe, but it still works out more expensive than day camps.

I also don't think it's to much extra work to adapt the program for multiple age groups.

As a short term care arrangement, payment should be upfront and in advance, just like a summer program would require. This covers your back regardless of attendance. I think if you are unavailable to provide care that there is no reason for them to pay for care, but if they take time off they need to pay. Offer the weeks as full weeks, not part time, but if there is a week they don't want then I wouldn't charge. Enrollment would work very much the same way as a summer camp would. Charge them a flat rate for the week, and then they can choose should they wish to attend less than five days a week. Other than that, my best advice would be to tell them to get a babysitter.