Too early for guitar? Not if she's truly interested in learning.

As someone who plays the aforementioned instruments (piano, sax and guitars) I'll add one small thought. Guitar, at any age, requires some stick-to-ness in the beginning, more so than I think other instruments.

The first couple of weeks the fingers can be quite sore from playing and the temptation is to not play each day as you should. If you and she elect to try it, I'd recommend a small-scale classical, the nylon strings make it a little easier, especially on little hands.

Once the fingers start to toughen up, it's a little more fun and a little less work, but until then...well, let's just say that I didn't learn guitar until my third try, at about age 19. Come to think of it, I didn't make it very far at age 12 when I started, though I was already playing a little piano.

Not to keep adding more information than you asked for, but if you can get your child interested and at all comprehending of music theory, that's the best thing I ever did. Music, at it's technical end, is mathematics. If you learn scales, modes, and such other nonsense it makes you able to: (a) get sounds from an instrument to match what your head hears, important when you're trying to write; and (b) with a solid knowledge of how music "works" you can learn new instruments quickly, as it becomes a matter of "how does this relate." Last year I learned basic mandolin and harmonica in  no time since all I had to do was figure out how the scales and note intervals were arranged.

Way more than you asked for, but learning to playing music gives back more than you ask for...

-jake