Yes, child pornography seems to be the one big outlier. However, even the Supreme Court ruled to expand free speech in the case of a borderline child porn case. See https://en.wikipedia.org/wiki/Ashcroft_v._Free_Speech_Coalit....

Most of the child pornography laws that are used in prosecution are actually obscenity laws, which are an entirely different class of strange laws. See https://en.wikipedia.org/wiki/Child_pornography_laws_in_the_....

Obviously this is probably the most egregious and disgusting form of criminality out there, so it's an important issue to deal with. I'm not sure what the proper solution is, but what we have now seems to work, given its extremely limited and well-defined scope.