The first warrant didn't cover kiddie pron. They went back and got a second warrant after they discovered it under the first warrant.
Going back to get a second warrant is a sham. If they can get a second warrant to cover anything they find that wasn't covered by the first warrant, then the whole purpose of warrants is defeated and you have no fourth amendment rights against unlimited search and seizure.
Although in your mind I am sure there is a kiddie pron exception to the fourth amendment right there in that living document, the US Constitution.