I can't tell you the going rate for *teenage* babysitters, but I know college students in my area routinely ask for $10-$12/hour. Also, we have a SAHM watch our son in our home for 3-4 hours and she gets paid a flat fee of $25, which comes out to $6-$8/hour. (I live in the Highland Park neighborhood of St. Paul).
One important thing to keep in mind is that "by the hour" babysitters get paid a lot more per hour than people who are doing all day daycare. Whether or not this is fair is a whole other posting.
IMO, this is between your daughter and the parents of the child. Your daughter now has her own relationship with these parents, and she, for the most part, should be the one to deal with them. I think telling the parents they are overpaying her would be ridiculous. I have never, ever heard of someone saying to their employer: "You pay me more than my friends make. Here, let me work a few hours for free, just to even things out."