'Nanny McPhee' Emma Thompson at Free Library of Philadelphia

As Nanny McPhee, the sensible antidote to Mary Poppins, Emma Thompson is snaggletoothed and warty, firmly dispensing gallons of tough love rather than spoonfuls of sugar. As herself, the English actress and author is pearly of tooth and creamy of complexion, mischief beaming from almond-shaped eyes.

Nanny McPhee Returns, a corking fantasy in which pigs fly and birds do not, hits multiplexes Friday. Last week Thompson, screenwriter and star of the sequel to the 2005 charmer, enchanted 100 children at the Free Library's Central Library. Stretching and snapping her elastic voice like an instrument, she explained how special-effects wizards made the piglets take wing. Know, however, that the crow perched on Nanny's shoulder is real. Thompson confided that she surreptitiously fed it so it stayed put.

The grown-ups in the audience were more interested in the disciplinary special effects of Nanny M, under whose magical influence quarrelsome brats stop fighting and start sharing. Given the inquisitive children at the library, the adults didn't have a chance to ask their questions.

But prior to her animated session with the youth from Kids for Change and Project H.O.M.E., the mother of Gaia, 11, considered how she might disarm a toxic tween. Not that it's necessary in her house. (She is wed to Greg Wise, Willoughby in Sense and Sensibility, for which she won a screenplay Oscar, mate to her best-actress statuette for Howards End.)

"Rudeness we don't like," she says, merry eyes mitigating somber precept. "If it happens, stern words will occur," she says. "We don't do punishment. Conflicts can be resolved in a quieter manner." In the unlikely event that Gaia did explode with "Mummy, you are such a" epithet rhyming with stitch!? Thompson would respond, "I suspect you're right. Anything else you want to tell me?"

Still, Thompson, who is also mother to Tindyebwa Agaba, 23, a Rwandan refugee whom she and Wise adopted when he was 16, is no stranger to the extremes of filial love and hate. On the corkboard above her writing desk at home in London are two letters from Gaia. "You're the best Mum in the universe," reads one. The other: "Mom, you're the worst mother. You just don't understand."