An excellent article about child porn is Schuijer & Rossen 1992. Especially interesting is the interview with Johnny, a boy involved in a child porn case.
Inciardi 1984 describes some children involved in pornography.
Child porn laws are very different in what they consider as pornography. It starts from laws which consider only the portrayal of illegal activities as child porn (Germany) and ends with considering almost any nude and even some dressed child picture as child porn. See, for example, Chatelle 1981 about the situation in the US.
There is the suspiction that the greatest supplier of child pornography are the governments - in undercover operations against pedophiles
There are a lot of attempts to characterize pictures of nude children as child porn. See some discussion: this, this, this.
I think they show the true nature of the anti-pedophile witchhunt - sexual repression. Indeed, by making (moreover by posessing) pictures or films with nude children no "harm" will be caused for these children. Thus, the usual justification fails here. Sometimes the "informed consent" argument is applied here too. The child cannot give informed consent to the publication of a nude photo. But why this has to be applied only to nude portrayal?
If such extension of child porn laws leads to more or less real sex with children is an open question. Indeed, some pedophiles may use nude child pictures instead of making real sex with children. But, if this substitute is also forbidden, it makes no sense to go this way.
The same becomes even more obvious if we consider computer-generated child porn. Here, we have obviously no harmed child. To forbid such forms of "child-porn" is simply sexual repression.