[Note: This is not a movie review but an assessment of some of the themes of the movie that might interest parents.]
"Harry Potter and The Prisoner of Azkaban" is the movie based on the third Harry Potter book. This movie is much darker than the previous two Harry Potter movies, "Harry Potter and the Sorcerer's Stone" and "Harry Potter and the Chamber of Secrets."
The movie opens with Harry, now about age 13 (but looking older in the movie), secretly reading a book on "extreme incantations" in his bed at night at his uncle and aunt's home. (The book version has Harry reading a section in The History of Magic which is describing how "non-magic" people in medieval times were afraid of magic but not good at recognizing it).
Harry seems to have poltergeist abilities. While he is in the kitchen listening to Uncle Vernon's sister, Aunt Marge, insult his parents, things clatter and tremble in the home. Finally, Aunt Marge begins to slowly blow up like a helium balloon. She soon floats away through open doors into the sky. Harry, while pointing his wand at an angry Uncle Vernon, tells him, "She deserves what she got." A short while later, Harry is being reprimanded by the Minister of Magic but told that Aunt Marge has been made normal and given a potion that will cause her to forget what happened. Harry knows he has broken a rule against using magic on Muggles (those without magical powers) and fears punishment, but the Minister merely says, "Harry, we don't send people to Azkaban for blowing up their aunts." Azkaban is the infamous prison for criminal witches and wizards. Here is another broken rule that Harry has gotten away with (in addition to the rules broken in the first two books). This follows the book very closely, although leaves many details out.
At Hogwarts, Harry is given the Marauders' Map, a map stolen by Ron's brothers from Filch, the school janitor. This map shows the location of everyone at Hogwarts as they move about, and a magical incantation erases the information so that those using the map can escape detection. The map later reveals the presence of Peter Pettigrew, a traitor everyone thought was dead, but who changed himself into a rat being kept as a pet by Ron. When Prof. Lupin leaves the school at the end, he returns this magical map (which had been confiscated) to Harry, although it is against the rules to have the map and Harry had received it as a stolen item in the first place. Lupin reasons, however, that since he is no longer on staff, he can do this without violating the rules. The wink at rule breaking in both the books and movies is continuous and pervasive.
Harry also makes use of the Invisibility Cloak to sneak into Hogsmeade, a nearby village. He is not allowed there because his aunt and uncle did not sign the permission slip. Another time, Harry uses the cloak to eavesdrop on a conversation in Hagrid's hut. And Harry uses the cloak yet again to beat up Malfoy, his nemesis, and Malfoy's cronies. Revenge is not presented as a bad thing, either in the books or the movies.
Prof. Lupin teaches Harry to conjure the Patronus, an entity or force that protects Harry from dementors. Harry must use his wand to do this. This scene is much more detailed in the book and resembles conjuring a thought-form, which is explained in the CANA article on the second, third, and fourth books.
Although divination class is presented in a comedic fashion by having a somewhat comical and inept professor teaching it, Professor Trelawney does at one point give a prediction when only Harry is present. This is also a scary scene, as it appears that Trelawney is being possessed while another voice speaks through her.
In the last part of the movie, it turns out that Hermione has been able to attend two classes at the same time because she has been going back in time using an advanced magic technique. Though using this magic is against the rules, Dumbledore instructs Hermione and Harry to do this so that Sirius, Harry's godfather, can be saved. Of course, no one else is to know about it. In the book, breaking this rule is described as violating one of the most important "wizarding laws" (p.398). Once again, rules are broken and magic is used to save the day (with the approval of the head of the school).
There is a very scary scene on the train to Hogwarts when a dementor, a ghostly deathlike figure covered in black robes, enters the compartment where Harry, Hermione, and Ron are sitting. Dementors, creatures that suck the joy and life from people, are used to guard Azkaban. The dementor causes Harry to pass out as he hears what he thinks is a scream. This scene is too intense and scary for young children, but it is only the first of several such scenes. Later, Harry tells a professor that he thinks the scream was his mother as she was being attacked (and killed) years ago by the villain, Voldemort.
Other scary scenes include Professor Lupin, a popular teacher who is also a werewolf, turning into a wolf and chasing after Harry and his friends. There is also a large scary black dog following Harry, large numbers of dementors floating down from the sky and attacking Sirius Black and Harry at the edge of a lake in a very chilling scene where it appears that Black and Harry are dying, and a scene where it seems as though a beloved pet of Hagrid's is being executed.
Due to the darker content and scary scenes, I do not think this movie is good for children under 12. My reasons for concern for older children seeing it include the nonchalant attitude about breaking rules by Harry and his friends, and the facile manner in which they carry this out; the revenge theme; and the spells and other occult practices that are part of the school's curriculum and the movie's story.