Don't take the littlest kids.
Harry Potter and the Half-Blood Prince is the darkest film in the series. That does not make it bad. But it should make parents pause and think before they let little kids watch it.
Those who read the book know the critical event in the story, the sad conclusion to it. I will not spoil that here, but that enough is sufficient to make parents of children under, say, 13, to watch it. But there are other things I can talk about.
For most of the Potter films, the character of Draco Malfoy (Tom Fenton) has been a stock, cliched antagonist. He's done the kind of "villainous" things that The Misfits did against Jem and the Holograms, that Tom did to Jerry, that Snidely Whiplash did against the Wacky Racers. Not this time.
While everyone was ooh-ing and ahh-ing over how the three principal characters were growing into young adults and starting to have genuine romances, Malfoy has a genuine conflict. He's only been playing evil up to this point. In Half Blood Prince he has been ordered to fulfill the duty of a Death Eater and perform a truly evil act. And for the first time in the series, he has a conflict of conscience, one so powerful you can see him torn apart by it.
More than anything, Fenton has the look of a Hitler Youth. Not one of the smiling, confident creatures in Nazi propaganda films, cheerful and supportive of the Fatherland. He looks like one of those youth in the last days of the war, thrown into uniform, handed a rifle and told to kill the invading Allied armies.
This is the main reason why I would not allow a young child to see the film, holding him back. Death and violence are everywhere in the film, video and videogame world. But this violence has something worse than stage blood or prop gore. It's violence of the spirit, of the soul, and young children simply can't comprehend or digest it.
For those who have read the book, a lot has been cut and trimmed. It had to be. The final book, Harry Potter and the Deathly Hallows, has been split into two feature movies, because J.K. Rowling wants it to be filmed intact. Most of the cuts will be missed only by completists. There is only one area of skimping that affects the drama of the story; given his importance in the film's denouement, the scarcity of Severus Snape (Alan Rickman) makes the film suffer.
The principal reason for film ratings is to protect children, so they say. In practice, they're usually used to keep controversial material off the screen that the rich and powerful would rather hide from us adults. But take my word for it, this film has material that I would not show to a young child, and anyone twelve or older would have to be a very mature and sophisticated kid before I'd take them to Half Blood Prince.