I can't even bring myself to harvest the Little Sisters in Bioshock. Even though doing so would make my character "stronger", I just can't do it. (plus, you can't get the best ending if you harvest even one) The game "rewards" you for harvesting them by giving you more ADAM (you use it to buy new powers and abilities). If you save them, you get less ADAM on the spot, but you'll get extra rewards every time you save X amount of Little Sisters. Thing is, there are way more powers and abilities than you can even equip at once anyway, so getting less ADAM isn't really a problem (unless you're playing on harder difficulties, but I haven't tried).
That being said... I think the reason people don't like immortal kids, specifically in games where you can choose to be a serial killer, has more to do with the fact that you can't kill them "because it's wrong". Unlike in Bioshock, where you
choose to either save them or kill them, based on your own judgment, and the game "rewards" you based on your choices. At least I hope that's all there is to it.
Bethesda doesn't allow killing children because it would change the rating of the game and take it off the shelves of most (legitimate) game sellers. There -are- video games allowing this sort of thing but they are largely banned.
They included children for a sense of realism (many complaints in MW and OB) but made them immortal to keep the game from being banned.
Bioshock says hello. It's only rated M.