I am with Charlotte Z.
I would have a good discussion with them, but make them aware that this is a business you are running and just like them, you need a stable income. My clients pay by enrollment, not attendance. They pay for their spot on my license.
Its actually fairly difficult to just fill in around a PT child, even if their schedule is set. You need a very specific set of hours to go around someone elses very specific hours/days. SOunds easy enough, but in reality its not. I currently have 2 PT kids. For the first 10 or so years I did daycare I never had PT kids. I love the kids, but hate the PT (one just went to PT this week..when I only had one part timer it wasn't so bad...not sure how I will like 2).
So one is only W, Th, F. The other is only M, W, Th. NOt so easy to fill around..just FYI . Also...I never take teacher kids. In my market they usually want summers off and I just can not afford that. Many around here will just take other school agers for the summer to make up the $$, but I do not take SA'ers. I am not set up for them.
But if you really love your work, you need to try to make it work, but on your terms.
Best of luck!