The Wolves of Willoughby Chase (The Wolves Chronicles)

Blurb

The Wolves of Willoughby Chase is a children's novel by Joan Aiken, first published in 1962. Set in an alternative history of England, it tells of the adventures of cousins Bonnie and Sylvia and their friend Simon the goose-boy as they thwart the evil schemes of their governess Miss Slighcarp.
The novel is the first in the Wolves Chronicles, a series of books set during the fictional 19th century reign of King James III. A large number of wolves have migrated from the bitter cold of Europe and Russia into Britain via a new "channel tunnel", and terrorise the inhabitants in their continuing hunting. Aiken wrote the book over a period of years, with a seven-year gap due to her full-time work; the success of this, her second novel, enabled her to quit her job and write full-time.
It is described by John Rowe Townsend as "a tale of double-dyed villainy, with right triumphant in the end."
It was adapted into a film, with the same title, in 1989.