horror - To what extent are child actors exposed to the violent aspects of the movie they perform in?

Base on the symbolic interaction there is a two-way influence between culture and cinema. As Herbert Blumer said: "IT is difficult to determine exactly the extent of this influence".

Mostly during shooting understanding abut specific shot or shots is completely different from what we see in the screen, because the deconstruction of motion and the behind the camera crew normally make actors very far from the Gaze from audiences that watch film in cinema.
So, we can give two separate answer to your question, in first part we need to understand about the different between effect of watching film and have a role in film, because all the effect that force lawmaker to make the limitation on the audiences are base on shared cultural structure and shared social structures that guid the their attention to percipient that cause to form a meaning in the audiences mind. So this process is different with filmmaking process, because in its process and actors involving in their role there is not the process of performance cause: percipient cause: meaning.

Blumer did a research on 200 boys under twelve years of age who were asked if they played in things seen in the movies. 75% answered Yes. Of 70 ranging in age from 12 to 14 years, 60% indicated that they played in what was seen in the movies. Among a group of boys between 14 and 16 years, 25% admitted still engaging in plays reflecting the influence of motion pictures.
All these three groups consisted of boys living in one of the slum areas in Chicago (Blumer Herbert, 1933).So we can extend this result of interaction to the insider underage actors, and think that only law can not determine the reality of cultural perception. All the human interaction to the film even as actor or as audiences is different from the social limitation that create by the law and it is more related to the norm.(http://archive.org/stream/moviesandconduct00blumrich#page/12/mode/2up)

In other hand and related to the note that i write here from Blumer, the effect of playing or getting a role in such a film that you ask can manipulated the underage audiences, so it make a some social queer appearance on them.