You know... Some kids music is good music. Some kids music is more violent then most adult music. Have you listened to "The Cat Came Back" recently?
"The man around the corner swore he'd kill the cat on sight,
He loaded up his shotgun with nails and dynamite;
He waited and he waited for the cat to come around,
Ninety seven pieces of the man is all they found."
I doubt I'd let my kids watch a movie with a guy who blows himself up trying to kill a cat.
Really, though, Grimm's fairy tales and most nursery rhymes are horribly violent. Rock a Bye Baby has the baby falling out of a tree, for cripe's sake. With contemporary music, I"m more concerned w/ drug and sexual references that aren't appropriate for my preschooler than w/ violent ones, although I'd certainly prefer to steer clear of the more egregious violent ones as well.
Oh, and language--I'm not ready to explain or hear words like ass, bitch, pimp, etc coming from my 2 and 4 y/o's. As my mom likes to say, let's save something for later.