Could it be considered illegal? Yes, but the problem is that they are already seeing girls their age naked most of the time anyway. It isn't illegal for two 13 year olds to have sex, but it's illegal for one to look at an underage child on the internet because you can never prove WHO is looking at the child porn. It could be the father for all we know.
Then another issue is child porn in general. Child porn has to be filmed or photographed by someone doesn't it? Which makes it illegal in the first place since generally that "someone" is an adult.