Nanny McPhee/WMG

Nanny McPhee is the spirit of the Brown childrens' mother

 * Nanny McPhee had to disguise herself with the imperfections as a way to conceal her true identity. It would explain the way she talks to Aggie when she first tells them to go to bed. As well, it would explain why after her transformation, no one calls her Nanny McPhee. She also gives a nod to Mrs. Brown's chair.

The Nanny McPhee films take place in the Disney movie-verse, just further down the timeline.

 * The extremely colourful first movie, to an almost absurd degree, and the vague location could place the film in the country several of the Disney films seem to take place in. In the second movie, the frequent references to a 'war' without actually identifying what war or even who the enemies are.  which would indicate that it takes place in the very vaguely defined Europe of the Disney Animated Canon.

Nanny McPhee is a Collective Identity...

 * ...of possibly thousands of mystical nannies deployed whenever and wherever they are needed, only to dissipate after their job is done. Thanks to Celebrity Paradox, they all look like Emma Thompson to us, but in fact look like the ugliest possible thing to whichever family they are sent to, then leave looking like the most beautiful. The military system may be an indication of how long this has been going on - they are literally Older Than Feudalism.

Nanny McPhee knows Mary Poppins in some way or another.

 * This one isn't even hard to imagine, they're both seemingly ageless magical nannies that show up when things are dire, have people who remember them from previous appearances in their life, and are very effective. Even if it's by reputation, they should know one another.