A sensible approach to children's exposure to violence in games (and, I might add, the rule can be applied to most forms of entertainment):

Lego [believes] that good-versus-evil combat "is at the root of children's play scenarios, and [that it] is an important part of a child's exploration of the world" ...

Chris Anderson, editor-in-chief at Wired magazine, explains further:

The Lego Company ... has a policy of not producing toys that replicate 20th century weapons. "You can have swords, and you can have laser guns in space, but no actual 20th century guns," Anderson says. So [Chris's] four children can play games like Halo, since it contains only futuristic, fantasy war, where you're killing only green- or blue-blooded aliens. The same goes for Roman swordplay titles. "But it clearly walls off Grand Theft Auto."

Of course, essentially this is the argument about whether children should play cops-and-robbers or cowboys-and-indians transposed into the 21st century.