Nurse Matilda by Christianna Brand
Christianna Brand was the pseudonym of Mary Christianna Milne (1907-1988), best known for her series of novels featuring detective Inspector Cockrill, and for the Nurse Matilda series of books for children, which were illustrated by her cousin Edward Ardizzone.
Later made into the Nanny McPhee films starring Emma Thomson, the Nurse Matilda books feature the hideously ugly Nurse Matilda, who is employed as a governess by Mr Brown to manage his many and very naughty children. Nurse Matilda uses her magic and forcible personality to teach the children how to behave. Christianna Brand wrote two sequels; Nurse Matilda Goes to Town and Nurse Matilda goes to Hospital.