S.L.
I have been the mother having to pay for day care I didn't use. It stings but i knew it was only fair. In essence, she did receive something for her money. she received the ability to bring her kids on those days if she could. You held her slot. As Dawn said it is business. I would remind her of the agreement and explain that you are not able to fill the spots she has held for her children on a random basis and therefore operate at a loss for the days you don't get paid. If the agreement no longer suits her then she can make arrangements elsewhere.