I think your hours have to be your hours for all clients. Otherwise it opens quite a can of worms, as it has in this case. I can totally see you not wanting too many kids if those aren't your hours, but I can also see the parents thinking that if others are paying the same price, why can't they have access to the same hours.

I don't think some families can have privilleges that others don't.

At best, I hope the family there at 7 is paying more and then your can explain that this option is only available for 1 child due to your limitations, but they can be put on a waiting list for this extra service....