how to get more money when you've already accepted less?

I've been a nanny for about three years , started a new position for a new family 6 months ago I watch a two year old boy and live with them rent free, I accepted 800$ a month to only realize I should be making about 600$ a week. now I realize it would be a big pay difference but the duties and tasks I have on hand everyday were not in the agreement , about a day or so more throughout the week i am watching not only the boy but his brothers (9&&11)and two of their friends(5&9) then sometimes the two years olds little girl friend who is three . I'm the maid around the house yet was only signed up for lite house work granted the mom does all their laundry, I'm the taxi driver. I don't mind all this I'd just like to be rightfully paid, the mom is home always great woman , my time was supposed to be divided on the two year old but realizing I'm catering to his needs as much as the mom's she doesn't drive so I'm constantly driving her to work the kid and I are in the car for about two hours he'll take a nap puts us at 3 critical to his development I have taken plenty of courses in college high school and interned for child care places while attending high school completed many hours . I also took on taking care of their dog . How do I go about getting far more than what I'm receiving