There is nothing wrong with having children that are able to die in a game. Simply because they exist does not mean the Devs wanted you to kill them, it means they had better things to spend their time on than making sure you wouldn't kill a set of pixels.
In a game where your whole goal is to kill children, it would depend on the context, but if they're just there and don't need to be killed, there is no problem whatsoever.
As for child killing context, Bioshock. You kill kids in it (If you choose to). Sure, they may be 'possessed' kids, but you have the option to cure them and make them normal, or kill them. I don't see anybody complaining about that - and that's FORCED upon you, and you are encouraged to kill them by someone. Invulnerable child systems seem to be companies making a big deal out of nothing. They go to extra effort to prevent something that few in their intended audience will care about. If its a possibility in an open world, or fits in with the context, there should be no problems.
In a game where your whole goal is to kill children, it would depend on the context, but if they're just there and don't need to be killed, there is no problem whatsoever.
As for child killing context, Bioshock. You kill kids in it (If you choose to). Sure, they may be 'possessed' kids, but you have the option to cure them and make them normal, or kill them. I don't see anybody complaining about that - and that's FORCED upon you, and you are encouraged to kill them by someone. Invulnerable child systems seem to be companies making a big deal out of nothing. They go to extra effort to prevent something that few in their intended audience will care about. If its a possibility in an open world, or fits in with the context, there should be no problems.