DO NOT MAIL ORDER A GUITAR!!!
Play one. Find a neck that fits the hand on a platform that makes the sound you need.
DO NOT MAIL ORDER A GUITAR!!!
Play them. Find a neck that fits the hand. She will know it when she feels it.
The neck between the nut and the 18th fret is the musician/instrument interface.
To a lesser extent, the bridge area.
A wide nut (1-3/4") on a thick neck may add to sustain but is frustrating with a small palm span or short fingers.
Conversely, a spider fingered basketball-palmer can get tripped up and tangled on a skinny little mando-style neck.
Sit down and play them until a neck is found to fit.
A cello player will know this. She'll find what fits. She's probably got the G and C chords down... maybe even barres. Get her hands on a variety of axes.
This is like trying on unsized boots.
Do not mail order a guitar for your child. Please.