Try the whole instrument
An electric guitar differs in body, weight and neck from an acoustic. Let the child try it seated and with a strap. Check that the first frets are reachable and the strap carries its weight comfortably.
What else is needed?
Amplified sound requires a suitable amplifier and cable; unplugged practice is possible but much quieter. Ask a shop how to connect it safely and keep volume low to protect hearing.
Must a child start on acoustic?
No. An instrument they enjoy and can play comfortably is a good candidate for a first guitar.