S.J.
Have you thought of a mandolin? It's a little smaller is tunable, and has a similar sound to a guitar but not quite as full bodied. It plays the harmony lines in most music but does a great job being the melody as well. My Mother started out with a violin at the age of 10ish and then transfered to a guitar as she made it to her teens. Then in her 40's she foudn the mandolin and has been completely entranced by it. I don't recall seeing her as happy with the guitar as I see her with her mandolin. I also recommend the mandolin because the neck is smaller and that allows it to be easier for smaller hands to reach around and make the chords.
Good luck making a decision. It is very hard to decide what to get.
S.