This is just a base to go from: My SIL pays $3 per kid per hour for her babysitter ($9/hour for three kids). I think that is a bit on the high end. In another situation, I get paid $30/day for an 8 hour shift watching one boy. Babysitting is and always will be an underpaid job.
As for a mother's helper, I would think that $2.00/hour is appropriate and you provide snacks/meals for your helper. You are providing her training for babysitting, and you are there to assist with the kids. If she does a good job, you'll know that you can count on her for babysitting jobs when she's a little older.
I hope that this helps!