My 11 year old son and I went to see this movie on the strength of the Disney TV trailers that really gave a totally false impression of what the film was about (it seemed perhaps it would be a great fantasy adventure similar to 'The time bandits', 'Narnia' or the recent 'Stardust'). In fact the film is more like a standard US teen movie, with typical school yard, home-life and school bus scenes. However the quality of the movie is simply in another league to most teen flicks, and it's got some visually stunning moments (not unlike quality films such as 'Alice doesn't live here anymore' and 'Fried green tomatoes'). It's got an intelligent script aimed squarely at an older kid audience, a fine young/adult cast, and is simply a good movie.
It's not for most, if any, under 9s though as it deals with late preteen/teenager childhood friendship and school problems. It has also got quite a few pretty children's fantasy sequences which my son happily took to be real, Narnia style - and it's not that scary, although it is emotionally intense. I don't want to give away any more of the plot though, just try it for the family as it's ideal to rent or perhaps buy. I can't see many kids watching it often though, as its power works best on first viewing - unless someone ruins it by giving away the storyline. That said, it's likely this will be a childhood movie they will remember.
The book, and film, was based on the author's son's real childhood friendship with a girl schoolfriend (although they were both eight at the time and not twelve as with the character's in the movie). I don't think the themes are unsuitable for older preteens - I enjoyed weepies Old Yeller and Bambi at 5 or 6 and in some respects the movies final scenes are quite uplifting. Besides the storyline is dictated by real events, not a Disney scriptwriter. It might be a good idea to watch the movie with youngsters though, not only because it's a good film, but also so that you can talk about it afterwards. My son and I really enjoyed 'Bridge to Terabithia', even though it was almost the exact opposite of what we expected given the Disney promotional trailers [and it's not really fair to blame the film or it's creators for that].