If I had a nickel for every time a parent asked me that question, book in hand. "Appropriate" is such a subjective term. Clearly what I think is appropriate for my own child to read (anything she wants) is not appropriate for others. But when an earnest, well-meaning parent asks me if a certain book is appropriate, how can I possibly know what words are acceptable--what scenarios, what characters, what thoughts? Librarians make judgement calls every day about books and where to place them in the collection or whether to buy them at all. But there is no Appropriate Ruler that librarians use to judge whether books are suitable for the varied patrons that cross our paths each day. We use common sense, professional assessment, and a healthy dose of "different strokes."
I had a patron this evening tell me that she has no time to read her own books because she is so busy keeping up with her daughter's reading, to make sure that everything she reads is "appropriate". Oh my goodness--what a pointless motivation. Of all the dangers there are in this world, a child reading an "inappropriate book" is really the least of them. Just let them read!