Here are some web sites that specialize in movie (and other media) reviews with respect to what's appropriate for kids:
But now I want to put in a plug for actual common sense rather than leaning on web sites or other people to tell you if it's appropriate. I'm always amazed how often I hear people complain about a movie whose objectionable bits are exactly what you would expect from that genre, director/writer, MPAA rating, or even title of the film, and in those cases I wonder if the complainer has experience watching any movies themselves.
You don't need to see the whole film first to know if it's appropriate. How about:
- Watch the trailer online -- this 2 minute investment can give you a pretty good idea if it will be scary, violent, or sexy (because those movies WANT to make it very clear from the trailers that it contains such things, to get people to see it).
- Use IMDB to look up the film and see what other work the director and writers have done -- maybe they are things you've seen and liked (or not), and you can read the comments and reviews about those previous movies from the same creators.
- Look in a newspaper (or its web site) -- major papers almost always have reviews when a movie opens that not only discuss the film in detail, but also generally point out which bits might be an issue for kids.
- You may disagree with MPAA ratings (I think they are too permissive with violence and gore, too restrictive with fairly tame nudity and sexuality), but they are relatively consistent. Considering a PG-13 movie? It'll probably have a bunch of people die and more swear words than you expect from a kid's movie. Don't be surprised, it's meant for teenagers.
Finally, resist the temptation to see things the weekend they open. By the second week, the "buzz" will be well established about what the films really are (not just the pre-release messaging the marketers wanted you to hear) and you can make a better judgment. Maybe even somebody else you know has seen it and can tell you about it.