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

I have worked with children on horror films/thrillers and, having found no official guidelines from SAG other than payment policies such as Coogan's Law, I have usually employed a number of tactics.

Firstly, I'll go over the scene and storyboard very carefully with the child's parents/guardians (although this isn't always constructive as many parents will let their child do anything to get them on screen ;)).

The simplest way I have found is to show the child the effects in play; we look at the props, the make-up, we play with the other actors, and I always use a sweet, edible blood and call it 'juice'.

Sometimes our shooting ratio doubles as we end up having to cut more often when the kids or other actors start giggling during a particularly gruesome effect.

The previous answers, regarding angles, solo shots and clever editing are spot on.

A fun example from "The Shining":

Because Danny Lloyd was so young, and since it was his first acting job, Stanley Kubrick was highly protective of the child. During the shooting of the movie, Lloyd was under the impression that the film he was making was a drama, not a horror movie. In fact, when Wendy carries Danny away while shouting at Jack in the Colorado Lounge, she is actually carrying a life-size dummy, so Lloyd would not have to be in the scene. He only realized the truth several years later, when he was shown a heavily edited version of the film. He did not see the uncut version of the film until he was seventeen, eleven years after he had made it.

The editing, filming techniques, and set attitude can seriously change the perception of what is actually going on. In various horror movie "behind the scenes" (last I can think of is Sam Raimi commentary on Evil Dead), it seems like casts for horror flicks are very upbeat and everybody is having fun. With many horror films it's only once the scenes are cut, music is injected and packaged that it finally becomes scary.

If this is true, then any child exposed to certain situations could perceive what's going on as a "game." A great example could be a scenario from "Child's Play." A director can say, "Hey, I'm going to throw you this doll, let's see if you can catch him."

While I can't speak for all parents, but some would definitely be on set to make sure that the child in question isn't overly exposed to "uncomfortable elements." However I can't speak for Linda Blair on "The Exorcist."

I have also worked on film sets and I concur with @Nobby. I think a larger point, though, is that situations are generally only “scary” when actors are in character. For example consider Halloween. An adult dressed like a corpse and covered in bruise makeup and fake blood isn't scary to most children if she’s chatting and laughing and walking around as if nothing was wrong. It’s only when she gets in character, acting as if in pain etc., that it begins to seem scary. Then consider that on a typical film set, the mantra is "hurry up and wait"—the camera isn't rolling for the majority of the shooting day. Even on the 10% or 20% of the time that the camera is rolling and the actors are in character and so on, there are still constant interruptions (between takes) when everyone resets and breaks character again. If anything, it’s harder to get kids to actually act scared than the other way around, when they’re surrounded by dozens of adults (the crew) who are completely at ease the entire time.

And this only really applies to people. Professional prosthetic makeup these days can be incredible (think Star Trek) and can look completely real even in real life inches away—but monsters and other entirely fake things often look laughably fake in real life. Just think of what a demon mask looks like in a costume store; that’s what movie monsters look like in real life. They only become scary on screen due to lighting and movement and sound effects. And non-computer-generated ones are either very still or operated by puppeteers, which makes them even harder to get scared by.

Just to share one related example: Linda Blair in the Exorcist did not understand what she was doing when she was told to act out the masturbation scene. It was only years later that she figured it out. So it's quite possible that child actors are often not quite aware of the exact nature of the scene they're acting out.

Source: Linda Blair herself.

I'm almost certain it comes down to parent permission and what they are comfortable allowing the child to see when working with the directors. Some are naturally more lenient than others while I'm sure some are only allowed on set to film their scenes and they never get to see what they filmed after post-production because of it being too graphic.

(The rest of this is speculative, take it with a grain of salt)

As far as specific laws, I know there must be some laws that prohibit a parent from explicitly forcing your child from watching disturbing content. Viewing alone or under parent supervision is not against any law that I can think of.

It comes to what the child agrees to do and if the parent will allow it as well. A director of a film will try to not to put the child in a vulnerable position in filming where the child feels uncomfortable.

If a child is not comfortable with what they will be exposed with (such as mock rape and murder scenes), the director can not under law be allowed to use that child for that particular scene.

It is also about safety too. If a child feels like they are protected from any real hazard, so be it.

The child also needs to know that the scene is not real as well, so psychological and mental considerations are needed.

In the case of blood and stabbing, it depends on the age and how well the child can cope with all the fake blood being around. When it comes to blades and threatening with a blade, the child needs to know safe knife handling (not that sharp knives are used).

In movies such as Annabelle, the children are exposed to death, massacre, gore and blood. The director will not expose them to too much of this if the child does not feel like they can cope with the extremities of the scene, then the director cannot use them.

With the parents, they need to know what their child will be exposed to and what role their child will play in that scenario, and the directors know that parents can say no to the use of kids who are under 10 years of age.

Only if the parent knows what their child is going to do in that scene, if they are to drip or shed blood, lose a body part, or commit a serious crime, they have to be able to trust their child will not get the pleasure of the violence and turn their mindset to crime. This is how dangerous using kids can be.

The following is from this website:

Have you seen the red band trailer for Kick-Ass [very NSFW]? How can a movie get away with a child actor spewing such filthy language? —Pocky, Los Angeles

You speak of Chloe Moretz, who catsuits up and plays a li'l purple-wigged assassin fond of the C-word, among other profanity bombs. She is a deadly vigilante who goes by the moniker Hit Girl, and her dad is played by Nic Cage. She filmed it at or around age 12, and this thing is very much rated R.

So will there be any last-minute injunctions or outraged, crusading lawyers trying to shut down the premiere on April 16? Well...

...for the record, I have no idea what Gloria Allred will be doing on the 16th of April, but in general, movies like this are plenty legal and not likely to raise much of a fuss. There is no law disallowing children to cuss in films.

As long as the child's guardians are A-OK with everything—and the underage Moretz doesn't try to sneak into a theater to see the actual film—the show goes on.

"When it comes to R-rated films and foul language, if the guardian approves—which the guardian must and sometimes sign waivers—then the child can be on the set," says Joann Perahia, whose teenage twin sons, Alexandre and Philippe Haussman, played Russian twins in the disaster film 2012. "It is all the parents."

That isn't to say that a kid can do anything on a movie set. Laws protect children from certain flavors of exploitation, particularly of a sexual nature.

According to Toni Casala of the social networking and advice site ChildreninFilm.com, a child cannot, for example, perform a sex scene, even if the sex is simulated. The sex must instead be more implied than explicit, or a body double must be used. That is, that is what child pornography law says; no telling what producers technically get away with on a set, especially if a desperate stage parent gives the A-OK and the on-set tutor doesn't bother to call an authority.

In the barely released movie Hounddog, a then-12-year-old Dakota Fanning performed in a rape scene without a body double. There is no nudity in the scene; the scene is very darkly lit and only Fanning's face and hand are shown.

Still, several Christian groups claimed the scene violated child pornography laws because it appeared to show a child in a sexually explicit situation. Prosecutors in North Carolina, where Hounddog was filmed, reportedly reviewed the movie, interviewed crew members, producers and Fanning. In the end, they found that while some people might find the film "disturbing and distasteful," there was no evidence that the scene constituted sexual activity under North Carolina law.