When kids are starting off on guitar they can usually fit comfortably into a student size guitar at about age 7. If they are smaller you can tune a ukulele to match the top 4 strings of a guitar. Nylon strings are recommended as their finger tips are not tough enough yet to press down on thin sharp steel strings. It is only the high E and B string that need to be changed out.
I often get asked if it is ok to start on an electric as it is often presumed that one graduates from acoustic to electric. It is really up to the individual. If all you want to do is shred heavy metal licks, get an electric. But if you are interested only in Spanish or classical finger-styling then a wider bridge nylon string guitar is the order of the day. Semi-acoustics with a cut-out I find are best for most guitarists who only want one guitar but wish to rock out with the band one minute then sit around the camp-fire the next.
As there are so many ways to approach the guitar and certain approaches work best with certain styles of music, I help students by focusing on those approaches depending on their musical goals.