For the first half of this movie, the children in the audience were completely on board, laughing when the children on screen were covered with mud and various kinds of animal poop, delighting in seeing them naughtily fighting with each other and then, when Nanny McPhee (screenwriter Emma Thompson) stamps her magical staff on the floor, each fighting himself. By the time the piglets were doing an Esther Williams-style synchronized swim number, the kids in the theater were extremely happy.

And then something happened that took the movie in another direction and the audience enthusiasm evaporated. This sequel to the 2006 original places the character inspired by Christianna Brand’s Nurse Matilda stories in a WWII setting as Isabel Green (Maggie Gyllenhaal) is trying to keep the family farm going with the help of her three children while her husband is fighting in the war. It is quite a struggle, especially because her brother-in-law is doing everything he can to make her fail so she will have to sell the farm. He has to pay his gambling debts or, two female enforcers tell him, they will remove his liver. Isabel also has to care for her niece and nephew, sent out of London to keep them away from the bombing during the Blitz. They are snobbish and selfish and there is an instant war between the cousins.

Enter Nanny McPhee, all in black, with a body like a linebacker, two enormous moles, a snaggletooth, a jowly chinline, and a bulbous nose. She explains she has been sent by the Army and she goes to work, banging her staff and bringing on the magic to teach the children five lessons. When she is not wanted but needed, she must stay. When she is wanted but not needed, she must go.

Nanny McPhee teaches the children to stop fighting and to share and cooperate. But then things get much worse when they get some very bad news and they must show resolve, courage, and faith before she will be no longer needed.

The movie is very uneven in tone and in quality, with charming nonsense colliding with what appears to be devastating tragedy. Children young enough to enjoy the silly pratfalls will be uncomfortable and possibly upset by discussions of death, war, and divorce. There is something jarring, even in a fantasy film, about children having to defuse a bomb as the adults are helpless. The timing is off so that even some of the comic set-pieces fail, like an extended bit about disappearing pens and a gruesome all-female hit squad who wander in like extras from “Sweeny Todd.” Thompson is always magic on screen, but here she is more wanted than needed.