I think one of the important things to remember is that, right now, no matter how high tech a game may be, it still looks incredibly unrealistic. Kids without any pre-existing psychological conditions may have trouble differentiating between a game and real life, but most kids shouldn't be affected. Having said that, the fact that the military is promoting certain first-person shooter war games as a sort of preliminary training is a little concerning and may indicate otherwise.
War games - hi-tech or otherwise - have been something children have played forever. Whether it's playing with wooden swords in the yard or playing Halo 4 on their Xbox 360, playing at war has been happening for centuries. It doesn't affect a child's natural development unless they have completely replaced social interaction with playing games all on their own - and even then, I wouldn't say it makes them more violent or desensitized so much as it makes them socially awkward and unable to function normally in a society where social interaction is heavily featured.
Assuming violent video games are desensitizing children in 2013 would be like assuming violent sword games are desensitizing children in 1567. How the games are played have changed, but they've always existed in some form or another.
Yes, the purpose of violent videos games is to train future soldiers. Whether boys fought with wooden swords, cap pistols or with drones the purpose is to desensitize. Round targets were used in the Civil War for this purpose. Teach no enemies.