I would have to say that an XR200 might be an excellent choice for a starter bike. The seat height should prove to be no problem, and the power and gearing should prove to be quite forgiving. It will be quite a bit heavier than your KX though.
My brother has a 97 XR250, and I would love to ride it more often, but I'm a 2-stroke fan as of this moment. The engine braking tends to throw me off quite a bit.
Also, beware of the "I'm power hungry!" syndrome. She may want to upgrade very soon after she gets used to the bike!
Edit: The XR 200s are also quite cheap and easy to find (at least where I'm from).