"Cheese Pizza" was a way of calling out and warning other users of pictures that were sketchy and of suspiciously young looking girls without running afoul of filtered or banned words on some bulletin boards and then forum/social media. It was picked up as slang for cp by "edgy" teens and eventually became so common that the pieces of shit that were posting the questionable stuff started referring to it that way. The same thing happened with the pedo bear.
You don't have to have been a pedo to have run into pedo shit in the early days of the internet. People didn't want that shit on their computers and in the early days browsers would cache every image on a page just in case it needed those images again later. So, when you did stumble across this shit, you let others know so that they didn't have to deal with it and manually purge their caches.