There is not that I have found a definitive list of citeria, only that a child avie is one who is portraying themselves as a youth. Truth be told thats not very helpful in the gray areas for an owner of an adult sim who has to make judgements over whether the really short avie is a child or a really short woman.
I ahve heard some use a height indicator as most kid avies are under a certain height.....I also look at the boob area.....but sometimes it really jsut boils down to your best judgement.