This is similar to this question on frame size, however that is addressing it from a road bike standpoint.
My wife is 4' 9", and I am interested in getting her a mountain bike, as she has expressed interest in hauling the kids back and forth to local parks, etc, in a trailer. We had a child's bike previously, that was...clunky at best. It doesn't need to be full suspension (Truthfully doesn't need to be front suspension either, although I suspect there aren't any of those around any more).
The frame size charts that I've seen all stop at 4'11" for the shortest, with a recommended frame size of 13-15". Will that be sufficient, or will I need to possibly look at kid sized mountain bikes? Her body morphology is normal (No overly short legs with larger torso, etc), so everything is in proportion.