Do we even have to say it? Glorification of violence, terrible attitudes towards women, celebrating criminality, unapologetic rampant consumerism, drug-addled, status-obsessed, hedonistic.
Sure, you argue, it's a window into our culture, perhaps even a therapeutic release in such a restrictive environment. But explaining this context to a 10-year-old is about as meaningful as trying to explain quantum physics or continental philosophy.
Obviously, plenty of kids grow up listening to rap and turn out reasonably well adjusted; it would be ridiculous to blame hip-hop for violence in society or drug addiction, even if it serves as a walking advertisement for both. But defenses that argue the music is harmless don't really wash.
It's the outlaw appeal, the rebellious posturing and indulgence in the forbidden, that makes hip-hop intriguing to kids in the first place. Parental guidance strongly suggested.