It's a game. I can kill anyone and anything. Drawing arbitrary lines in the sand just feels stupid.
That said, even though I can kill anyone or anything, I rarely do if I am presented with the opportunity.
(I kill things by accident sometimes, which is annoying.)
I get the odd impulse to go on an insane killing spree if a game will let me, or purposely do things the game mechanics imply are bad...
But that feeling doesn't usually last beyond a few minutes, at which point I go back to being almost obnoxiously good.
I remember games where you have the option of knocking out, rather than killing your enemies...
I did that too just for the fun of it, a lot of the time.
Thing is, it's not real. And I don't see what's so horrible about killing a fake depiction of a child compared to killing a fake depiction of an adult.
It's still killing. (And I don't really get the mentality anyway. Aside from the fact that most kids are defenseless compared to adults, and it's messed up harming people who don't stand much of a chance of defending themselves, I don't really see it as being any different.)