Oh, I LOVED that bit in Fallout3! You obliterate an entire town, murder everything you come across for fun, but lo and behold, the little bundles of joy may murder and cannibalize one of their own, but can't be harmed. Talk about immersion breaking bullshit...
Here's my compromise: if a game features an open world (Fallout, Oblivion), there should be children in them. If you kill one, your wanted level (or whatever it happens to be) skyrockets, and nobody wants anything to do with you unless you make some costly amends.