As a magical governess charged with whipping seven unruly children into shape in Nanny McPhee, Emma Thompson serves up spoonfuls of an ingredient that's not nearly as sweet as sugar.

But having also whipped up the script, she makes the movie go down easily.

Thompson returns to screenwriting for the first time since her Oscar-winning 1995 adaptation of Jane Austen's Sense and Sensibility, this time using the Nurse Matilda books by Christianna Brand as her inspiration. (Though comparisons to Mary Poppins will be inevitable - or to another iconic Julie Andrews role, Maria from The Sound of Music - this nanny reveals herself to be a unique creature, not at all what her looks initially would suggest.)

Nanny McPhee swoops down out of nowhere to help a widower (Colin Firth) who's busy managing his mischievous brood while searching for a new wife. The children reluctantly learn life lessons and find that behaving themselves and functioning as a cohesive family unit isn't so torturous after all.

It's a story that could have been treacly, and director Kirk Jones' film threatens to go in that feel-good direction from the start, with its focus on the deceased wife's empty chair in the cozy den of a rambling Victorian. But Thompson and Jones' twisted, dryly British sense of humour (he previously wrote and directed the delightfully morbid Waking Ned Devine) often makes Nanny McPhee surprisingly funny.

The esteemed supporting cast also does wonders to elevate this beyond just a colourful, high-energy children's movie, including British stage veteran Derek Jacobi, Imelda Staunton (an Oscar nominee last year for Vera Drake) and Angela Lansbury in her first on-screen film role in more than 20 years.

But the child actors - led by Thomas Sangster from Love Actually as the eldest, Simon - do seem to have a blast bouncing off each other. And how could they not? They get to perform monstrously cruel and destructive acts, the likes of which have driven away 17 previous nannies. A bulletin board hidden in the closet, covered with garish, voodoo-doll likenesses of each frightened woman, is a wonderfully dark touch.

The household's pretty young scullery maid, played by Kelly Macdonald, is the only one who sees the innate good in these kids, which is relevant for reasons that are pretty obvious pretty early. But that's OK.

McPhee won't be scared off as easily as her predecessors; her appearance is scary enough in itself, and distractingly so. Her bulbous nose, unibrow, warts and misshapen figure indicate she's not a woman to be messed with, but it's her stray front tooth, hovering like a fang over her bottom lip, that too often draws away attention from what's happening on screen, including Thompson's own dialogue.

Thompson always carries herself with an intelligent, regal presence, though, regardless of the setting. Here, she conveys substance and subtlety with just the slightest nasal grunt of amusement. And the running gag in which she suddenly appears in a room, startling whomever she's come to see - but insisting, "I did knock" - is good for a laugh every time.

McPhee helps the kids navigate their pompous Great Aunt Adelaide (Lansbury, who also appears to have dipped into Nicole Kidman's prosthetic nose stash from The Hours), who threatens to cut the family off financially if Firth's Cedric Brown doesn't remarry. And she agrees to look the other way as they torment the ghastly Mrs. Quickly (Celia Imrie), the wealthy widow who's their potential stepmother, if they agree to suffer the consequences of their actions. (See? This is one of those little life lessons.)

She accomplishes all this with the bang of her gnarled walking stick on the ground, an act that sets her spells into motion. Unfortunately, the resulting visual effects look jarringly cheesy and jerkily sped-up. But considering that Nanny McPhee gets most everything else right, watching a stupid-looking dancing donkey isn't the worst-tasting medicine to swallow.

Two and a half stars out of four.