You know i love Hitchcock movies, but he made some really excellent works like "Psycho" or "I Confess", and also some movies that are okay, but rather average, such as "To Catch a Thief" or "Strangers on a Train". This one is the first. It is an excellent piece of art, one of my favourite movies ever.
The movie would be rated PG for thematic elements if rated nowadays.
There is not much inapproriate for children:
(SPOILERS may follow:)
-There is much pressure executed on Mrs. de Winter, as she is always compared to Rebecca. It may be too intense for younger viewers to see a main character does not feel comfortable all through the movie.
-A character asks Mrs. de Winter, if it wouldn't be better if she (de Winter) jumped out of the window (she almost does).
-A deadly accident and a hidden body is a main topic, but it is never seen and it is not disturbing.
-A castle is burned and a woman is inside.
(END OF SPOILERS)
The movie is suitable for children 10 and up. If you want to show your child older movies, this (and "Citizen Kane") is the movie to start with.