Any world built upon the bodies of children must be a hellish, nightmarish, or dystopian world. Sometimes it's one in which the player takes on the role of monster.
For a medium that revolves so much around killing, it's sad that so few games show us the realistic consequences of violence. That's probably why there are so few kids in games and why they're always supernaturally protected from player created chaos: no one wants to sensationalize child murder. There were no kids in the “No Russian” level of Modern Warfare 2, and you can’t kill kids in Fallout 3 even though there are many in the Capital Wasteland. Despite this trend of avoidance, there have been a few recent and semi-recent games that deal with the killing of children explicitly and implicitly, and it’s no coincidence that they’re all horror games.
In Dante’s Inferno we’re attacked by unbaptized babies in Limbo. While the babies have long blades for arms, which make them look inhuman, the rest of their body looks like that of a normal infant, so even though they’re clearly dangerous, it’s unsettling to have to kill them. Their role is important in characterizing Hell as a horrible place. Dante’s Hell is governed by a strict system of rules regarding sin, and if you don’t follow those rules, you’re punished. The most frightening thing about Hell is that it makes no exceptions for race, gender, age, status, or any other kind of context for the sin committed. A sinner is a sinner and will be punished, even if the sinner is an infant who simply hasn’t been baptized.
The use of infants as enemies is effective here because they appear so early in the game. We see that the punishment for any sin is relentless and we know that Dante is a sinner himself, so we’re encouraged to fear for his well being. Dante may be trying to rescue Beatrice, but now we have to wonder if anyone will rescue Dante. Hell is built up as a greater antagonistic force that threatens Dante directly, not just through Beatrice. This plays out later in the game when, after Beatrice is saved, Dante remains in Hell to fight Satan. This more personal fight is foreshadowed by the enemies that Dante fights in Limbo.
Dead Space 2
Like the unbaptized babies, seeing children and infants turned into monsters reinforces the merciless nature of the necromorphs. We see proof that we’re up against an enemy that takes no prisoners; every fight must be a fight to the death. This knowledge that combat is necessary for survival is one of the major gameplay traits that separates Dead Space/2 from the classic survival horror games in which it is expected that players will run past many enemies to conserve ammo. You can’t (or at least shouldn’t be able to) run from the necromorphs; the mutated children are proof of that.
Their presence also gives us a glimpse into what life was like on the Sprawl before the outbreak. This was indeed a place where families lived, a community in every sense of the word. Without the Pack or Crawlers to firmly establish this fact, the daycare/school would feel like an arbitrary addition to the world.
In Bioshock, the Little Sisters are not so inhuman that we can shoot them without a second thought. They never attack us for one, so we can’t argue our actions were in self-defense, and the only inhuman thing about them is their big, alien-like eyes. Yet no matter how unsettling their eyes may be, the mere fact that we have a choice to “save” them means that they are not entirely lost to whatever evil force has afflicted them. They aren’t monsters.
Instead, it’s the player who takes on the role of monster. We can kill the girl to gain more of the precious Adam that fuels our power in Rapture or save her and gain very little (in theory). The choice is meant to reflect Andrew Ryan’s philosophy of individualism: sacrifice the girl for to grow stronger and stop Ryan or let her go and fight Ryan with our own strength? Ryan betrayed his own beliefs as he took greater control over Rapture in his war with Fontaine, eventually becoming the monstrous dictator that we see in the game. The question then becomes, will we make the same choice? Turning ourselves into a monster no different than a necromorph.
These examples certainly aren’t realistic depictions of violence, but they’re not sensationalized either. In each case, the deaths of children underscore the greater horror that we face throughout the game, and in doing so, they contribute to building up a truly frightening vision of a world. This kind of world building only works for horror games, though. Any world built upon the bodies of children must be a hellish, nightmarish, or dystopian world. But if that’s your goal, then showing the world’s effect on kids is certain to drive that point home quickly and firmly.