Yes, the ACLU supports child pornography, because they support all kinds of terrible things in the name of freedom. The ACLU's position is that there is no right and wrong, until the point that someone claims they are offended. To them, wearing an American flag shirt on Cinco de Maya is offensive, but child pornography is not.
Although the ACLU did represent NAMBLA in a 2000 wrongful death court case, that does not necessarily mean that the group supports 'child pornography'. The group has also supported a variety of hate groups on the basis of freedom of speech. Supporting a group's right to freedom of speech does not necessarily lend support to the things they're saying.