Everybody has specific ideas about the material they're comfortable with their children reading. Maybe you're okay with exposure to some swear words but not others. Maybe violence is out, swearing is tolerable, but you're concerned about how your child is exposed to different social situations than what he's used to. Maybe you have a 7-year-old who's worked through everything you can think of written for that age group, and you need to feed that child's voracious reading appetite without giving her nightmares. Maybe you just want to be prepared for questions your kids might ask you as they work their way through a book.
Bookalachi can help. This site was created for this very purpose and includes parent reviews of children's books. They also tell you how they approach reviewing books. They consider content in these categories: language, sexual content, alcohol/drugs, violence/scary/disturbing, social/family, and religion. Their intent isn't to censor books, but to provide parents with tools to make the best decisions considering their own family's values.
Here's a taste of what this site offers. Bookalachi will tell you that Harry Potter and the Sorcerer's Stone that language-wise the book is a Level 1. Level 1 books include words like "hell" and "bastard," but not four letter words. It also lets you know that in the book, the reader will see an adult kiss another adult on the cheek. Under alcohol/drugs, the site tells you that there is an "item described as looking like a ‘cigarette lighter’" in the book; also adults drink brandy and wine and in several cases it changes their behavior. A number of factors are listed under violence/scary/disturbing. The social/family category tells you that Harry's parents are dead and that he is raised by an Uncle and Aunt who actively dislike, neglect, and abuse him, and mentions a few other things. The religion entry points out that the book features "witches, wizards, ghosts, poltergeists, and fortune-telling. It also includes this quote from the book, "There is no good and evil, there is only power, and those too weak to seek it."
This is a great resource for parents who like to know the content of the books your children read, but who don't have the time to read it all beforehand.