I don't think it's a game for a 9 yo, it doesn't even matter the language, the whole setting is disturbing. The side story which ilustrates Mombasa last 24h and the Convenant invasion is told with very dark flashbacks and a story of a young girl who escapes a sexual abuse from a police officer. Plenty games around for a 9yo, Toy Story 3 is great, Lego Harry Potter is awesome, ODST is the most mature Halo yet, definetly not for children. That's my opinion anyway.
If you get all the Audio logs you know about the story I was talking about, it's not only about the Covenant, it's also about what humans transforms into in extreme situations. Get all the Audio logs, that litle story is almost totaly not-Halo.
But yeah..., if you just want to shoot stuff in the face I guess the story doesn't realy matter anyway. To each its own.
If you ever get interested in what the heck am I talking about, and don't realy want to play the game to its full and live what developers wanted to describe, here is the full transcription of all the Audio log files, and Sadie's sad (probably pun intended) story: http://www.gamefaqs.com/xbox360/954261-halo-3-odst/faqs/57855
[...] SADIE: Where are we going?
KINSLER: The police department has an emergency bunker outside the city limits. We'll be very safe.
SADIE: That's very kind of you, Commissioner. But I can't, I'm sorry. If you would please take me to the city center--
KINSLER: Such a serious young women. Just like your mother... 'New Mombasa's most passionate public defender.'
KINSLER: Too bad her last client really was a sick, murdering bastard.
KINSLER: (sliding across the seat closer to Sadie) She had your eyes...
SADIE: (into her Chatter) Vergil, stop the car!
KINSLER: As police commissioner it's my job to uphold the law. And I'm afraid this city has very strict, personal privacy statues.
KINSLER: My car is off the grid, Sadie. As far as Vergil is concerned, you just disappeared...
[SOUND: Sadie slaps Kinsler's face]
KINSLER: (laughs) Assaulting a police officer. That's a serious offence.