I can browse the Internet for a long time before I see an archive full of hard-core pre-teen porn unless I go looking for it. It is difficult to browse the Tor .onion network without seeing at least links to this. Sometimes users land on it by mistake. Try it.
Tor gives the impression of being a sea of kidporn with little islands of interesting stuff, especially when you factor in the cognitive fact that human beings give greater weight to negative stimuli.
I haven't cruised .onion sites in a long while, but I just tried and I didn't come across anything bad by accident. I don't even understand your assertion that you can end up on any .onion site by mistake, since the URLs are impenetrable.
You raise a good point. There is a difference, however; the Internet is actively monitored and ISPs respond to/work with the police to reduce this kind of content. Can the same be possible on something like Freenet?
What about PGP? and cryptography in general? you can't do an algorithm that don't encrypt CP..
There is no way to stop a particular kind of content, and yes: technology could be used for bad things! you should already know this.
Even if every package were monitored&filtered and encrypted messages were illegal steganography could be used to circumvent the censure. So, it's impossible to stop the distribution of CP, we must accept this.
The real problem here is child abuse, and people should focus on this. Sometimes I think that actually our society don't really care about the children: it's more like they just don't wanna know and that's why the common answer from the govt is "filter the Internet".
IMO the best we can do is think how we can find the child being molested on certain pic, not trying to figure out some magical way to stop distribution, because if we keep trying this we're just wasting the(ir) time...