I have had good luck with care.com, not so much with sittercity.com.
I also have preferred girls in their mid-twenties (and had excellent experience). Slightly older than college age, but not moms. I want someone with maturity, the scheduling availability I need, high energy, and willingness to take direction and follow instructions. Mid twenties, at least in this area, consists of a lot of people who are doing an additonal degree, are schooling part time, and many have teaching related degrees already because of the terrible market for new teachers around here.
Quite frankly, I don't want another mom with her own ideas about raising/treating children, nor do I want her kids as part of the package - I don't want extra kids at my house, causing wear and tear, I don't want exposure to extra sickness (in either direction - her kids make your kids sick, or your kids make her kids sick and she can't can't some to work!), and if I'm paying someone for childcare, I want the care of MY child to be the primary focus. YMMV on all these points.
In your specific case, you may want to up your pay, and also adjust your vocabulary - the way you describe the position isn't really a nanny job, but a part time sitter. If you are using the word "nanny" in your ads, people may not be following through because they are expecting something different.
Good luck, it is sooo tough to find childcare you are comfortable with, no matter what your expectations.
Jane