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 confortable 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; Depending 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 extremeties of the scene, then the director can not use them.
With the parents; They need to know what ther child will be exposed to and what role their child will play in that scenario, and the directors knowing that parents can say no to the use of kids who are under 10 years of age means no.
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.
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.