Last Fall I bought my 11 YO daughter...
.. a Giant Boulder SE. It is a bit on the heavy side. The alternative to a heavy bike would be buying a XS "real" bike, which she would outgrow in a year's time.
This bike was $30 more than the bottom of the line bike, but that extra $30 bought a much better front shock. She's ridden it in Moab and many rides around here. It works well for her. If she shows real interest in MTB, I'll get her something real when she stops growing. If I wanted to I could easily shave 5 lbs off it by changing the cranks/pedals, wheels and seatpost, maybe brakes.
I'll never pass up an opportunity to post a shot of one of my kids riding:
John