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.