Porn with children can not be made without raping a child. That is why it is banned and obviously laws protecting children from sexual exploitation is constitutional. Child pornography can not be conflated with typical pornography made with consentual adult actors. The child pornography issue is raised only to muddle an otherwise crystal clear 1st amendment issue. This isn't to say that I think pornography has any redeeming merits, or even that it doesn't present some very clear dangers, but my opinions don't change the clear meaning and principles of the constitution.
You have made a reasonable point that in child pornography, there is the additional problem of child (what do they call them? actors? stars? models? rapees?) being utilized in the production. The Japanese have solved that particular problem with child rapee CGI's.
But there is the additional problem that, as I said in #35, the First Amendment was arguably intended to protect all controversial speech and publishing, i.e. the conveyance of ideas. Cognitive content. That did not include, even then, sedition, which is a form of treason; and I would argue it did not include porn, which is arousal-based (which is to say, comparable to the purveying of a harmful drug) --- and not communication-based.
I think it could be objectively defined, too. And on a more objective basis than, "I don't know how to define it, but I know it when I see it."
I don't think the "prurient interest" criterion is utterly useless, but going a step further, surely some University can monitor the sexual arousal patterns of experimental subject-viewers, program that basic info into a computer, and thereafter use the computer make a reasonable assessment of whether the material in question is physiological-arousal oriented rather than intellectual-cognitive oriented.