What is the best way to handle a situation when you know you're being taken advantage of?

So I offered to babysit for a co-worker or mine during the holiday time. She has her daughter for two weeks and I wanted to help because I know she doesn’t see her very often. When she asked me how much I wanted her to pay me I told her whatever she could afford. She told me it wouldn’t be a problem because her ex husband was letting her keep the child support she pays for this month. Anyway, I’ve been watching her little girl for some time now. She picks her up an hour later than she says she is going to and drops her off atleast 30 minutes earlier than we had planned. I end up watching her little girl 16 hours a day. I was trying to be nice because I am a single mom and I have had so much help, but I would never be this inconsiderate. I don’t get much sleep as it is, and because she threw her daughter on me last minute today I only got 2 hours of sleep after working a graveyard shift. It also wouldn’t be so bad but the girl is, bluntly put, a brat. I love kids and I am good with them but she is very mean to my son. I have caught her tripping him, hitting him, tackling him, throwing him down, grabbing toys from him… You get the point. I understand she is an only child, and probably taking her parents divorce kind of hard, but the mom won’t help me out at all. I have told her how she treats my son and she just goes “Oh yeah?” I don’t think she’s ever tried to talk to her daughter about it.

I guess all of that wasn’t necessary. My point is she hasn’t paid me yet, and I hate to sound that way, but as much as a burden as it has been I need something to make me feel better. Her daughter doesn’t go back home until the 6th and I don’t know if I should say something to her mom about being on time. I just don’t know what to do!! Help!