Two things...
Firstly, if I were to guess based only on real world possibilities, I'd figure most if not all of those kids would be orphans. In a post-apocalyptic wasteland mommy and daddy might fall victim to any one of the myriad of enemies in the wastes. Super mutants, raiders, vampires, rad scorpions... etc.
Second... this is the 7th most disturbing thread I've ever read. A mod, you say? That enables you to kill the children... Erm. Well. Wow. My brain hurts now.
you obviously havn't played the first two fallout games . . .
about the history of little lamplight, i think they were originaly a schoolclass stuck in the caves, and they hadn't any contact to the vault-inhabitants directly.
one of the vault-technicians heared them, and thought it was his own child that died during the vaults experiments. the sound of laughing children drove him insane over the loss of his child.
it's not explained how the caves get "restocked", but since everyone turning 18 has to leave the caves, i think that those grown up children then send their own children back to the cave, so they can grow up in a save spot.
though nothing of this is explained ingame, but i think it would make sense.
[Edited by Killa_B, 11/13/2008 8:47:37 AM]