What does not fit well for her? Seat height? handlebar reach? If that really was a 12" frame size MTB 26" wheels you bought, your 4'11" girlfriend should do fine. Remember that, properly adjusted on the bike, she should barely be able to tiptoe toreach the ground when sitting on the saddle. I see far too many people riding with too low a seat height and no where near reaching full leg extension on downstroke.
My wife is 5'1" and rides a 14" frame size MTB, Motobecane 700HT, with 26" wheels. The bike fits her quite well and she has enough clearance over the toptube to be comfotable and safe. At first, she complained that the handlebar seems a little too far out front. I explained to her that even though an MTB sits more upright than a road bike, you still have to lean over a bit and place some body weight over the bars. After a while, she got used to it and now like the way it rides.
You may want to help her look at "comfort bikes" which has an even more relaxed riding position than MTBs, with higher handlbars. I really don't think she needs to be looking at kid bikes.
Last edited by Volfy; 05-02-07 at 07:23 PM.