I have raced motocross for almost 9 years and I have owned hondas, yamahas and kawasakis. The kawasakis are by far my favorite bikes and I have had the least amount of problems with them. I started racing 125's when i was probably about her height or a little shorter and I could control the bike fine. I doubt she needs the power of a 250f because she will probably never see a track so i would get her a crf230f and they aren't too expensive I think a brand new 07 is right around $3500. Just get her suspension set up for her body weight and her height, she will be able to control the bike much easier if it is set up for her.
