Not a "mountain" type frame, but have you looked at the Giant FCR series? My 10 year old daughter has the 2005 FCR 4, and loves the heck out of it.
Basically, it is a road-type frame with V brakes, flat bars, trigger shifting, and 700c wheels. Once we got her on that last Xmas, she's been doing 40 mile rides with us. It is lighter than a MTB framed bike, and the 700c wheels/tires really help her go faster for longer distances before tiring.
Here's what one looks like:
"Love is not the dying moan of a distant violin, itís the triumphant twang of a bedspring."
S. J. Perelman