Try before buying
Let the child test several guitars while seated: does the body rest on the leg and can they reach the first frets without stretching? Compare neck thickness, weight and string height. A trial lesson with a loan instrument can help decide.
Getting started at home
A tuner and safe place to store the instrument are useful. For amplified electric sound, you also need a suitable cable and amplifier. Involve the child in choosing the sound.
Should I buy a larger guitar to grow into?
Only if it is already comfortable now; an oversized instrument can make the first steps unnecessarily hard.