Getting the right size and type of guitar for a young beginner is very important. Generally I recommend buying an acoustic guitar with nylon strings, as these are much easier on the fingers! A 1/2 size instrument would be suitable for an age range of 5 – 8, and a 3/4 size instrument for ages 8 – 12.
It is also helpful to have a decent, padded gig bag, a capo and some spare strings.
These are some of the more useful articles I’ve found on the web.
The most important thing is the guitar is a comfortable size to play, and the strings aren’t too high off the fretboard. Online reviews of instruments are a good resource, but it can also help going along to a music shop and trying a few instruments out.