I took my 8yo daughter to see this last night and I was pleasantly relieved that she was OK with it, but I would hasten to say that many 8yo's would absolutely not cope with the scary and intense scenes in the movie!
Having read all the books and watched all the movies myself, I was firmly persuaded that the film franchise was for kids 12 and over ... but then my 8 year old came home with the first HP book in May this year and she devoured it ... and the next ... and the next! When she got up to the Order of the Phoenix I thought she'd give up but she persevered still.
My husband and I discussed her newfound passion and decided that we'd use the HP books as a foundation for some discussions with her about the themes of friendship, loyalty, love, pride etc. We also said she could see the DVDs, and watch DH at the cinema if she finished all the books by Nov 18.
And again, I stress, this movie isn't for all kids. Basically, if you know your child can handle scary or intense scenes then they should be ok with this movie. The movie is very true to the book so you pretty much know when something scary is about to happen and if you're squeamish, you have plenty of time to cover your eyes! If you haven't read the books then you'll struggle a bit, and fail to make sense of Nagini, and also the camping scenes which can be a bit intense from time to time.
The critics are all deriding the film as being 'too dark' ... which makes me roll my eyes because they're only using the material that JKR wrote! If any of the critics bothered to read the books they'd see that the film isn't departing from the HP tradition at all, it's actually an excellent film adaptation of a fantastic novel. The filmmakers have done a great job of elevating the mood from time to time with some hilarious and touching scenes.
A bit has been made of Harry & Hermione's clinch (the imagined scene that Ron sees in the locket) and quite frankly it's nothing my child can't cope with. It's (a) obviously an imaginary sequence designed to terrorise Ron and (b) so stylised that H&H look almost like cartoons (think a gothic Ken & Barbie) and (c) very brief - the big 'kiss' was a blink-and-you-miss-it.
If your youngster has READ THE BOOKS then they shouldn't have a problem with it as long as you know they can handle (brief) scary scenes, you warn them that a scary bit is coming and they've been to the toilet before the film (it's long!).
It might pay to flick thru the book beforehand and refresh your memory on the first scene at Malfoy Manor, and then again in Godric's Hollow ;) In a nutshell, I'm glad I didn't make my daughter stay home, she simply loved it.