Hmm, I'm actually in agreement with Google here - how do you remove "child porn" and distinguish it from, say, Dakota Fanning's performance in Hounddog, or a family's pictures of their baby having a bath and splashing around in good fun? The thing about child "porn" is that in some sense the pornography can be *in the eye of the beholder*! To a pedophile, an innocent picture of a kid in the bath could probably be arousing, but for the family that took it, it's just cute. These are just a few examples, there are much more challenging gray areas out there that I don't care to delve into. The point is it'd be really, really hard to do this in an automated way, and if you tried to empower certain people to do it, well then you just have the morals of a few being enforced on the many, and that's not good either.
Bottom line the best way to fight child porn - and anything unsavory - is to find the source. Trying to cut off the means of distribution is difficult at best, particularly when said distribution conduit is *largely* used for many other legitimate things (the Internet).