I'm sure this is familiar to some of you, but I need to type it out anyway.
Bethesda released a new Fallout game, which is a good example to use, although the same complaints can be aimed at a wide variety of games. In Fallout 3, you can't kill children. In the earlier games, you can. In fact, it's something that can happen by accident, especially if you're having a firefight in the middle of a village. Using plasma cannons and missiles. Which happens fairly often, actually.
Non-gamers (and probably most gamers) will stare at you in horror when you say that the game is not as good because you can't kill children. "You want to kill children?!"
Tch, that's not the point. This is not a matter of programming children to be killable. This is a matter of programming children specifically to be exempt from death in a world where you can kill anything and everything else. It's roughly the equivalent of going to see Blade Runner or some other beautifully atmospheric movie, only to have a corner of the screen filled with a cheerful cartoon animation doing the Macarena. For the whole movie. But it gets bigger when the movie is at its darkest and most atmospheric. After all, you wouldn't want anyone to get depressed by the dark and atmospheric nature of the movie!
Let me see if I can explain it in another way.
You want to live in that game's world, at least for the moment. You want to live in that world, but THE CHILDREN AREN'T LIVING IN THAT WORLD. They are exempt from the world. They are immune to its dangers, pressures, etc. They are the equivalent of a comic book character breaking the fourth wall, and that's not always suitable.
So, YES, if you can't kill the children, it's no good. Not because killing the children is good or even because I want to. It's because if they're immortal god-beings, it completely ruins the immersion. Destroys the reality of the game. For the sake of some little old lady's heart?
It's rated M, lady. Kill the children.