Who needs fathers?
I have to admit, I walked out on this movie about half way through for one simple reason: it was anti-father. The movie opens with a single father and his brood of kids. The children are wild and out of control because, the movie suggests, fathers are inadequate for taking care of children.
As a father taking care of children is certainly a recipe for disaster, movie logic demands that a woman be brought in to fix the situation for him. The titular character fits the bill because, well, she's a woman and therefore just has to be a better caretaker than any old man. There are also competing love interests for the man because, again children need a mother: a mere man is inadequate.
Thus Nanny McPhee teaches us a valuable lesson: fathers serve no function in a family except to write checks. Raising children is a woman's job, and men should just stay out of it and not have any pretensions of adequacy. This lesson comes as quite a shock to us fathers who are more actively engaged with our children than their mothers, but this is Hollywood speaking, and when has Hollywood ever steered us wrong?