Child porn is a complete red herring argument by the government. Like someone else mentioned above, it's next to impossible to quantify things like child porn and child porn prosecution. It's not like there is an open market of child porn floating around. Shits already beyond under the radar.
These proposed laws are about file sharing, nothing more. They throw in things like child porn, and drugs, what have you, to make it seem like they are all the same. They are not. (obviously).
They share the commonality of being illegal, but that's about it. Nobody, minus perhaps the ultra fringes of society, support any leniency for child porn. Most americans support some criminalization of drugs.
It's a smart move by the government really. Take two common villains (child porn, drugs), lump them in kids downloading music, and you can potentially glean a lot support from people who want to stop obvious evil shit like child porn.
Even thought sopa and whatever the governments second swing was, are dead in the water, i really doubt we're going to see these kinds of attempts go away any time soon. As long as people like the Motion Picture Association and the Recording Industry Association of America continue to fist fight the internet, these laws are going to get lobbied in every year or so for the indefinite future.