Nanny McPhee

Nanny McPhee (small c, big P), she's the main character in the Nanny McPhee movies.

She is an immortal witch and her job is to discpline bad children and turn them into good children.

Her work is "When you need me, but don't want me, then I'll stay. But when you want me, but no longer need me, then I'll have to go."