I've been considering women's specific frames for my youngest, in particular the Dawes Sheila on bikesdirect.com. They seem to have a geometry more forgiving to kids, usually with smaller bars and some of them have smaller frames still on 700's. The Sheila appears to have even it's smallest frame on the larger wheels with a nice low standover.
http://www.bikesdirect.com/primalstuff.htm
As a note, however, some women's designs are on 650c wheels. You'll want to avoid those, the parts are hard enough to come by when you aren't riding from one end of the Earth to the other!