I happened upon this website a few years ago while looking for something else:
http://www.kids-in-mind.com/
It's definitely got a Christian bias, but the way they describe movies, and how appropriate they are for kids is pretty objective, and allows a parent to know what's in a movie without giving away any/many plot details. Here's an example from the Captain America review:
VIOLENCE/GORE 6 - This World War II Era film includes many scenes of gunfire, military vehicles shooting cannons, soldiers firing on the enemy, falling bodies, men carried on stretchers, exploding grenades and bombs with smoke and fire; there is little blood.
A man flicks a cigarette lighter, blowing up a piece of equipment, and then shoots another man in the chest (we see three dots of blood over the heart and he dies).
They rate each film according to Sex, Violence, and Language "quotients" so you can see at a glance how much violent or sexual content has. And, being that it's such a conservative group doing the reviews, they tend to include things like kissing or hand holding, which many might not find offensive, but some probably would.