Although I agree with the majority of people that killing kids is socially unacceptable, isn't killing entirely unacceptable? Everywhere?
I've asked myself the same question as the threadstarter numerous times. And have also thought about all the new gameplay elements that open up for you when there are kids involved.
Think of GTA, there were schoolbus-look-a-likes driving around in some places. Wouldn't it be a good idea to drive a schoolbus and pick up children, then bring them to school. Or your protagonist has a kid, niece/nephew or something like that, which get's kidnapped and you're following 4 missions are to find out where they're hiding him/her, and who's kidnapped him/her.
I think there are a lot of new things to come by. And I know that it will never happen.
Too many people (like me) explore the game's limits. If you're driving a bus full of hookers, what will happen if you'd blow it up? Now replace the hookers with children, and you've got your reason why kids in shooters aren't OK.
Though I'd buy a game like that immediately, if only to support the balls of the developper/publisher.