I think you guys are missing the point...
he was actually asking IF child actors like Edward Furlong back in 1991 is allowed to see the R-rated movie that he was in...and of course, even if he was in it, then yeah of course he would know that there's gonna be alot of cussing and violence in it and he did cuss, as well so yeah he proably knows what's it gonna be rated before the rating system gives its a rating...in real life, Edward Furlong was actually around 17 in T2, even though he does look like a kid but hey...he's pretty short lol.
There are other child actors who did appear in R-rated films that has sex and nudity scenes and I'm not really sure if the child actors who appeared in their films are allowed to see that kind of part.
There aren't really many famous child actors anyway so it's kind of hard to come up with a perfect example of a film with a child actor that shows tons of drugs/violence and sex/nudity, etc...but I think they would allow child actors to watch films that only have cussing and violence but not the sex and nudity parts...lol