Some people are just idiots. If your child appears to be an idiot at a certain time in their life, then do not buy them such video games. If you yourself happen to be an idiot, then please do not use the online feature. Remember, age is just a number, but idiocy is for life!
This has been a public service announcement. =3
(Serious answer: I don't really have any strong opinions on violence in video games, but the thing I find kind of sad about this is that a lot of kids playing MW2 probably won't understand the themes of the game - i.e. the fact that the game is anti-war. I do worry that at a young age they'll interpret it as a glorification of war & violence, rather than appreciate its more complex message. But, then, there are much older people who would do that too, albeit in fewer numbers.)