I have an easy counter-argument: filtering anything, including child pornography, costs civil liberties. So it better have some positive effect. Fortunately for the sake of the argument, the filtering of child pornography is already in place in some countries. So we can just ask: did such filtering affected the number (or condition) of molested children? That's what it is supposed to be about, after all.
My uneducated guess is "no, it didn't". Anyway, a direct answer may be difficult, so we may want to ask related questions: Did such filtering affected the number of paedophiles? Did it affect the paedophilia market?
Again, my uneducated guess is "no" to both answers.
So, unless we can answer "yes" with an appropriate level of certainty to any of these questions, filtering child pornography probably isn't worth the resulting loss of civil liberties.