I dont think so, you are basically shooting aliens so in that respect it is just a realistic version of space invaders.
I think Halo 3 is a fantastic game, the best fps around and I think if younger people are going to play a shooter this should be it.
It is less violent, realistic or bleak than COD 4 or COD 5 and it simply isn't in the same league as say Grand Theft Auto in terms of language, sex references or violence also unlike in GTA you are a hero not a villain and are never asked to do anything morally wrong. I could get into the ins and outs of why I think the game is great but if I did that I'd soon have a bunch of fanboys trying to argue with me.
Putting aside the rating system if you are considering buying a shooting game for a kid then Halo is the best you could get them. If you are willing to let them play a game involving shooting people then you should be willing to let them play halo.
You can go on to Youtube if you fancy sifting through thousands of Halo videos or I'd use gametrailers.com if you just want to see the official videos.