It is important to note that while the children are sent to the country to avoid the air raids during World War II, the reader is not explicitly told this. The answer to this question must be inferred through the detailed description of how far removed the Professor's home is from the city and the fact that this was a common practice during this time.
The book is set during the height of the World War 2 air-raids on England (particularly on London). So, the children are sent to the country so that they will hopefully be protected from the air-raids. Even though Lewis wrote the book after World War 2 had ended, the book's publication came not long after the war, the devastating effects of which would still have been seen and felt in the country. So, a book which focuses on children being able to escape the physical and emotional elements of war was warmly received.