Please help with this issue!?

Ok I'll start first at; I'm a 27 yr old stay at home mom of 3 kids ages 11, 4, and 2. My husband works 6am-4:30pm Monday-Thursday with some Fridays. I have been babysitting for my best friend of 17 years, she has 2 boys ages 5 and 9. My husband and I have been fighting over her paying me. The problem? She doesn't. It was fine at first because it was a kind of agreement, if I needed $200 for a bill I could go to her and that was it but now, a year later, it's just not working. Her shift is always "changing" day - evening so I can't make plans, she works Thursday - Monday so I have extra kids on my husbands days off, another issue. We feed these kids about $75-$100 worth of food a week, yes he seriously added it up, and that's not including when we go out to eat this is just meals and snacks at home. I used to not have a problem with this but the last 6 months I haven't been getting child support from my sons dad so we been short on cash and her ever "changing" schedule is killing me. I mean I can't take my own family to do things on the weekend because I have to have the money for 2 extra kids to do it also, so we've spent almost our entire summer at home. what should I do? She has family in town, a brother but he works all the time and an irresponsible mother that can barely care for herself. But at the same time she works full-time and gets child support from both the fathers of her kids. My husband is constantly angry over this mess says she walks all over me "uses me".. I love her, she's been like a sister to me all these years but I'm starting to feel like he is right. What the hell do I do? Should I ask for pay or just stop watchig them? What's a reasonable amount to ask for a week for 2 boys? Please help I'm lost but tired of being broke. I can't afford to do for my own kids because I feel like I'm supporting 5 kids instead of just my 2.