Forget the trails for right now. Let her concentrate on learning how to use the bike. Let her get used to the shifting--the best way, I think (provided she's ridden a bike before and is comfortable with controlling the bike) is to just go out in a big empty parking lot or quiet streets with low traffic and experiment. A little explanation (either from you or the bike shop) helps too.
This is how I'd do it:
Once she's comfortable with the gears and the bike, see if you can find some packed dirt/crushed limestone paths or a large, relatively flat grassy area to ride around in. I'd highly recommend a mountain bike skills clinic if you have them in your area.

Encourage her.