The basis (at least according to the SCOTUS) on which child pornography fails a 1st Amendment test (remember, popular speech doesn't need protection) is because it makes those trading in it accessories after the fact in a crime (children under the age of consent cannot, obviously, legally give consent to what is being done, making it sexual assault). Now, things have gone overboard (a teenage boy was thrown in jail as a sex offender because he took pictures of his girlfriend in a bathing suit; the judge decided that the center of the picture was too close to the girl's crotch). And, while pornography involving prepubescent children probably should be in a worse class than pornography involving postpubescent, it currently isn't. Finally, there's a matter of a burden of proof; the feds are trying to make it the law that the burden of proof is on the possessor of the material that the subject is of legal age (note that the federal government has a past history of using unpopular targets to set precedents which they can then sic on the general population; for example, the precedent which allows the IRS to estimate tip income of waitstaff and taxi drivers was set on drug dealers and prostitutes; I for one have a problem with guilty until proven innocent, no matter how heinous the crime).
However, and the Supreme Court currently agrees, if no real children are involved, then the 1st Amendment takes precedence. Unfortunately, once laws become well-established, their bases are often forgotten. The drug laws were started on the basis of drugs (including marijuana) making people insane. When murderers started using the "I was smoking marijuana" defense, the marijuana laws in particular became justified on the basis that taking drugs tend to lead to people taking drugs; the circular nature of the logic being less important than the so-called "war on drugs" (Grant in the past has shown a rather interesting connection between the war on drugs and the comic industry; it's partially theoretical, but it meshes well with everything else I've read).
I think child molestation is despicable. But I find the government using child molesters to to subvert the law is even MORE despicable.