Albert Bandura did some stuff a few years back on the impact of television violence on children. Part of one study, if I remember correctly, showed TV "model" children beating a "bobo doll" (a plastic doll that stands back up after being hit). It appeared to show that children in the experimental group showed more "violence" toward a real bobo doll than children exposed to a "neutral" TV video.
You might check also in the general area of American Television Ratings - Children. My own emperical evidence suggests that TV material watched by children is mediated for "good" or "bad" by parental teaching both direct and by example. Children seeing whatever (violence, loving, caring, hate, etc.) on TV will be more influenced by what their parents do, less than say. Children will model violence on TV if they see their parents acting in violent ways even if the parents say that violence is bad. "Do as I say, not as I do," simply does not work.