I'm going with yes but only conditionally.
I've been around the world on several combat tours and in that time I have seen no distinctions between Child or man. Kids are just as deadly and adults when given the chance, so in an effort to increase the reality factor in gaming, why not include such an option? Now where I think it's conditional is things like CoD. Well written and well presented however you only engage adults...why is this? The middle eastern world does not share the same ideologies of "Women and Children First" so why should we excluded such a terrible truth from games? If people really want a fell for what combat is like, you should be exposed to all aspects of it, not just the "glamors" parts. Should GTA take this route, I think it would just be a means of shock sales, like there "accidental" inclusion of a sex tape. Just try to tell me that wasn't to increase sales and it did. Killing children in such an inviroment is senseless and tasteless, however put this concept into a more realistic setting, such as modern war, I think you could be onto something.
But that just what I think.....