After setting a babysitting rate can I reset it with the parents?

I started babysitting three kids and I set the rate at $5 an hour. But the second time I babysat these kids, they were REALLY REALLY hard. I don't feel that I was properly paid for what I had to put up with and I don't know if I even want to babysit them again. But I've only baby sat these kids 3 times. Can I reset the price ?