Children are generally seen as the epitome of purity and innocence in our society. They are highly protected and are thought of as our future. People generally have the feeling they have done nothing deserving of death and that anything done against a child is the vilest of evil. Of course with 3rd graders plotting to kill their teacher and 14 year olds beating each other up on youtube the lines blur more and more all the time.
Still killing children, or just having children, in an already controversial game would not be well accepted by society in general and I'm sure R* has enough to deal with just passing the content they already include.
Some of us understand this is only a game and nothing more but not eveyone sees it that way. Some people also do become affected by playing such games and act out in real life what they do in the game. Any game maker that makes such games is definitely walking a thin line. They want to give the gamers what they want but they also have to have it be acceptable to the public or it will get pulled off the shelves and they won't make any money.