The only real rule we have in our house is that if the movie is based on a book, the book must be read before the movie is watched.<quoted text>We did that with our kids too.
My opinion on this -- very much based on my memories of MY childhood -- is that most kids know the difference between "pretend" -- movies, TV shows, cartoons, etc.; and "real life" -- i.e., The NEWS!
The 7-year-old has both read all 7 Harry Potter books, and seen all 7 movies. We read them together as a family over a few months--taking turns reading each book aloud in the evenings then we watched each movie once we finished the novel. If you can't handle the content in the book, you're definitely not ready for the movie.