Compare side by side
Have the child play the same open string and fret-one note on both instruments. Which lets their shoulders stay low, wrist relaxed and guitar steady on the leg? A three-quarter model from one maker can feel different from another's.
Check playability
A smaller size won't help if the strings sit too high. Ask a shop to check an instrument that's hard to fret. Nylon strings belong on a classical guitar; do not put steel strings on a classical guitar.
Should I always size up for growth?
No. Avoid buying an instrument that is uncomfortable now in the hope it will fit later.