How much should I pay this sitter?

We live in the midwest, Illinois to be more exact, near the Mississippi River. Minimum wage is about $8 an hour here and I do not make too much more than that. We are hiring a 16 year old girl to "babysit" our 10 year old. The job will be about 4-6 hours a day 2-3 days a week. What is a decent hourly wage to pay her? I was thinking $3-$4 an hour, but I really have no idea what is customary and don't want to be cheap. Thanks for your responses in advance!