for example, with nicki minaj, for a child the most obvious thing would be the theatricality of it - the voices, wigs, crazy outfits.

there's tons of stuff i listened to as a kid where i didn't understand the lyrics, viewed the performance in a different way. i now look back and think 'wtf?' but in the end it didn't have a detrimental effect on my development. additionally, i was exposed to lots of other music/art/culture and always had a grounding from my parents in terms of what was right/wrong/acceptable/etc.

i totally see why you would not deliberately go out of your way to buy miley cyrus cds for your kid, but at the same time, if everything else is in order, exposure to some of that stuff probably won't do any serious harm.