Based on the novel "The Young Visiters (or Mr Salteena's Plan)", published in Britain in 1919 and written by Daisy Ashford who was only 8 years old at the time.
The source novel incorporated a lot of Ashford's mis-spellings, and the film does this also - in the title, and in some of the signs which can be seen in the palace scenes (e.g. 'Prince of Whales').