The existence of this thread proves how fucked up we are as a society. Granted, in fallout 1 & 2, you could kill little kids, but graphics were so low-tech and games weren't that big of a thing to bring public attention to.
Now, we have graphics that look more real than reality itself, it's obvious that REALISTICALLY killing children would cause extreme media uproar, calling this game "disgusting, disturbing, and outright wrong," and they would be right. Society has made children untouchable. It's why Chucky is more creepy that Jason Vorhees. The girl from the ring more disturbing that Freddy Krueger. Children are seen as a beacon of hope in a time of war, poverty, and worldwide despair. Destroying that beacon would either make people think that this game has crossed the line, or that our line needs to be redrawn. This would result in either a game that can't legally be sold, or games where we can have graphic rape in them. Hey, if realistically shooting a child in the head and seeing it be blown to smithereens is okay, then why not other disturbing and sick crimes?
In short, it's more of an unwritten rule. Currently, there are no legal laws in place that say that someone can't make such a game. This game, if made, wouldn't find a publisher. Six Days In Fallujah was dropped, and that just depicts adults killing each other.