Oddeus: Wasn´t there a cannibal perk in the Fallout games? Can´t remember how it worked. Maybe a mod was necessary to be able to kill a npc that wasn´t an adult.
dtgreene: It appears, according to a quick Google search, that such a perk exists in modern Fallout. I assume killing children is not possible in those games without a mod, seeing as how Bethesda went out of their way to prevent doing so in Skyrim.
(I consider Fallout 3 to be the first modern Fallout, as the gameplay style is completely different from that of earlier entries in the series, and I suspect most people familiar with the series would agree.)
Yes, there's a perk that allows you to eat people in the modern Fallout games. There are also hunters/scavenger that'll sell you "strange meat"—which is heavily implied to be human—in Fallout 3, and a nice lady that will offer you strange meat pie; if one wishes one could imagine that it came from a child, I suppose. Finally there is a quest and an item that allows you to eat humanoid meat in Skyrim. There're also vampires, but I don't suppose those count.