As Fallout 3 lead developer Emil Pagliarulo (pictured) points out, the game is M-rated. That doesn't mean that anything goes. In fact, Pagliarulo stated that during initial development meetings the team grappled with whether or not it should be alright for players to shoot kids' heads off. In an Edge column, the designer explains:
We're grown-ups, we can make and play something like this. We think it's funny, we think it's fun and people have been agreeing with that. We don't want to cross lines like killing kids (we actually never got as far as even putting kill-able kids in any builds of the game). For us, that was a line we certainly didn't want to cross, and we think that was the right decision. It wouldn't have been socially responsible, at least in the case of Fallout 3.
Just because one can, doesn't mean one should. Be sure to read the whole article in which Pagliarulo does a fine job of explaining how he and his team confronted the issue.