The reason I'm in favour of child-killing mods can be explained with my experience in New Vegas.
I was roleplaying as a psychotic, bloodthirsty character--not really what usually comes to mind when one says "evil", though, because he kills everyone, including people who contract him to kill others, and doesn't really discriminate by faction or standing or class.
So I came to this one point where I was massacring the Legion forces at the Fort, and I ran into some children. I'd been ruthlessly slaughtering everyone else here, and my character was a psychotic murderer---a cannibal, who'd help someone out and then chainsaw them to pieces while they were thanking him. So it only made sense that I'd murder the kids too, right? But---I didn't feel right about it. I'd gotten some moral twinges before while RP'ing as this guy, but--not like this. I gulped and held down the button to use my Ripper on the child, but---he screamed, ran away, and I didn't chase him. I cringed at his cries for help and hated myself for trying this. I just left the Fort in disgust. This is one of the strongest experiences I've had in gaming.
A week or so later, I wasn't RP'ing or trying to get immersed, just randomly gunning people down 'cause I was bored. I saw one of the kids and, since I hadn't been trying to get in RP-mode, just saw another bunch of pixels to wipe out. I opened fire and the kid ran away. I continued chasing him and firing and firing and firing and firing, but he never went down. It occurred to me that they'd probably made the kids unkillable.
And that ruined the previous experience because as it turns out there was never going to be any consequence for my action. My inability to kill the frightened child, even while absorbed in the role of a bloodthirsty killing machine, meant nothing because I wouldn't have been able to do anything of any significance even if I HAD been able to push myself over that edge.
So it's not so much that I want to kill kids. I want to be able to choose not to.