Mountain bike sizing for a short female 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.
Proper bikes do exist in kids sizes, in essentially all types. 24" wheels are now common, which makes things easier. Some examples:


Ridgeback dimension 24" (hybrid as you said you don't really need suspension) 
Specialized Hotrock 24" (hardtail)


Many of the major brands offer equivalents.  This is something where sticking to the big brands probably makes sense, as there's plenty of junk out there too.  These tend to be entry-to-midrange bikes (though there are exceptions) but unless the park is up a mountain or 10s of km away hey'd be more than adequate. Compared to the full-size equivalents you probably get slightly lower-spec gearing, but that can usually be upgraded quite easily/cheaply especially if done when the bike is being serviced anyway.  They're often in stock in big bike shops so test rides should be easy to arrange.

Compared to squeezing onto a slightly too-large bike (at a similar price point) you should get a lighter frame, and if you go for suspension it should be capable of being set up for someone of that weight.

A test ride is essential whether you go down this route or try for the smallest bike marketed for adults.  

And a tip: getting moving with a trailer is harder than without; jumping up onto the saddle every start isn't going to make it easy.