Uniting the fantasy elements of "Mary Poppins" with the sort of misbehaving tykes found on any one episode of reality series "Nanny 911," "Nanny McPhee" yearns to be a magical, moralistic family entertainment, but doesn't quite come together as it should. In adapting the "Nurse Matilda" books by Christianna Brand, director Kirk Jones (1998's "Waking Ned Devine") and screenwriter-star Emma Thompson (2003's "Love Actually") have created a potentially great title character in Nanny McPhee—a bedraggled, mysterious figure with warts and a misshapen red nose who comes to the rescue of a troubled English family—but they fail to ingratiate her with the people around her or develop her beyond a rough sketch. The same thing could be said for the film as a whole, which occasionally sparkles with a promise of becoming something more than it ever really is.

Since their mother passed away, the seven Brown children have become unruly tyrants, scaring off no less than seventeen prospective nannies. With their father, Cedric (Colin Firth), busy at his job as a funeral director and unable to keep them in line, much-needed help arrives in the form of Nanny McPhee (Emma Thompson), who shows up on their doorstep with surefire, albeit mystical, ways to make the children see the error of their ways. "When you need me, but do not want me, I'm here to stay," Nanny tells them early on, "and when you want me, but you no longer need me, I must go." As Nanny sets about to correct the siblings' behavior, she also comes as a savior to the whole family, who could use some help in coping with the death of their parent and wife. And, with Cedric being forced into marrying the rotten, money-grubbing Selma Quickly (Celia Imrie) by their wealthy Great Aunt Adelaide (Angela Lansbury), who is helping the Browns out financially under the condition that he weds, Nanny McPhee better work fast.

With copious sequences of destruction via out-of-control children and lame physical pratfalls, "Nanny McPhee" starts off by bringing back nightmarish memories of 2005's "Yours, Mine and Ours" and "Cheaper By the Dozen 2," but then settles into a pleasant groove once Nanny McPhee shows up on the scene. Her magic-laden methods of helping the children, including eldest leader Simon (Thomas Sangston), to learn the value of obeying the rules, respecting themselves and each other, and saying "please" and "thank you" are cleverly orchestrated in Emma Thompson's screenplay. Thompson is less successful in developing the seven children and their virtually nonexistent relationship with their father. Cedric is portrayed as a man who doesn't think twice about allowing others to raise his children, and when he finally experiences an epiphany about the mistakes he has made, it comes too late in the proceedings to make an impact and feels like an afterthought.

Acting behind a thick layer of make-up that renders her nearly unrecognizable, Emma Thompson makes for a memorable, well-meaning Nanny McPhee, but she is let down by herself—she is the screenwriter, after all—by never making the character into more than a plot device. There is also a questionable message sent out that should have been scrapped at the initial outline stages in which Nanny's physical blemishes and flaws begin to vanish with every lesson the children learn from her. It is an inconsequential story development that proves nothing and seems to wrong-headedly suggest that outer beauty is just as important as what's inside. Younger audiences, who have enough problems with their peers in trying to conform to unattainable good looks and perfect skin, don't need to see this, if even subconsciously, within the confines of an otherwise good-hearted movie.

The rest of the cast are fine with what they have to do, but are wasted in thinly conceived roles. As mentioned, Colin Firth (2004's "Bridget Jones: The Edge of Reason") has trouble bringing sympathy to Cedric, who comes close to being a deadbeat dad. As loyal maid Evangeline, who is clearly the person Cedric is meant to be with, Kelly Macdonald (2004's "Finding Neverland") is an amiable presence. Angela Lansbury (TV's "Murder, She Wrote") shows up in the scene-stealing supporting role of the Browns' hoity-toity Great Aunt Adelaide, and chews up the scenery with refined relish. All of the young actors playing the bad seeds in need of proper guidance are just okay, never really getting a chance to separate themselves as individuals.

Until the ending, which tries too hard to pluck at the heartstrings and ends up plucking none, "Nanny McPhee" diligently avoids sugarcoating its story. There is a tinge of darkness to much of it that would make Tim Burton proud—mortician Cedric, for example, is seen several times working on corpses—and the 97-minute running time races by so quickly that the picture doesn't have time to wear out its welcome. This, alas, may also be the film's most crucial downfall. "Nanny McPhee" has a sweet, non-cloying innocence that proves fetching, but not enough time is spent displaying the positive changes Nanny McPhee forms in the lives of the Brown family to make the experience more than a missed opportunity. When the finale arrives, and the children finally want her, but ultimately no longer need her, Nanny McPhee makes her inevitable exit just as promised. It's too bad that she hasn't made a big enough impression on either the family or the viewer to be missed.