No you mean why not a Bethesda game.. Because im sure there are games that allow it..
It's odd that in GTA, you never see any kids walking around...
Child slaughter in video games would very much be frowned upon because it is YOU the player, doing it. If it were in a cutscene and the big bad guy was killing an innocent little girl, it'd be acceptable because it was for the advancement of the plot like in a movie or in a book. But video games are interactive, and the media thinks that kids are influenced by the things they are able to do in video games.