A kids show that I can't rationalize how it ever got passed the censors is the old Cartoon Network's "Cow and Chicken". In particular the secondary character known as the Red Guy. 
(I googled that video, I didn't watch it through.)
I guess you don't remember Ren & Stimpy doing the butt-bump (or whatever you want to call it), and it would actually make a kissing sound everytime it happened.
http://www.youtube.com/watch?v=ZkiSYmn74osPretty sure there were things on the show that were even stranger, but I only remember this because the song was done to death.
As for how butts are acceptable in children's entertainment, I'm doing to take a wild guess: butts were a comedic device before they were ever a sexual one. I can't prove this though. I tried googling it. I found a million links talking about when breasts became sexualized. But I couldn't find a single one about butts.
