They are taking care of your children. If they have children of their own and want to take them on THIS say to a "waterpark" for instance, why should her children have to forego a trip because she is doing you a favor??
Now if it's a matter of economics (you can't afford said waterpark; used only as an example) then you may want to ask her to please postpone said trip because at this time you are unable to afford such a trip.
Even in a day care they ask that you pay $5 for that trip to the museum, etc.
Also, I do not allow my kids to take their friends anywhere without the knowledge that I will be paying. In saying that, if I cannot afford to pay for their friends, I politely and tactfully tell my children, your friends can come but they will have to pay because I cannot at this time.
I am a firm believer in open and honest communication with EVERYONE. If I'm broke...I tell them. If I am offended, I tell them. If I have a question I ask it without assuming.
Makes for a light and drama free day.
Sending good thoughts your way.