Babysitting & getting shorted money..

Madison
After having my daughter 2 months ago I decided to take some time off work. Instead i decided to watch my two cousins ages 6 and 7 for the summer while they are out of school. I had made arrangements with their mom (my aunt) to watch the girls 3 days a week for 4$ an hour per kid so 8$ an hour. She agreed. 
This week was the first week I watched one of the kids (the oldest isn't out of school yet) The first day I watched her 7.45 hours so I just rounded down to 7 being nice. The other two days it was 9 hours on the dot. So that equals out to 100$ for the week.
Well, she paid me today and handed me the cash and I didn't even think to count it then. Well she paid me 70$... I hate to be rude but I mean 70$ isn't really worth my time to bust my ass watching a extra kid with my newborn ( i'm a first time mom so i'm still new to everything!)
I guess i just don't know if i should say anything to my aunt about the money but i feel like i'm helping her out watching them for 4$ an hour where anywhere else is more than double that a kid.. I need some advice.