Now, they could have the children still evacuated because bombings if they lived on the Falkland Island during the 1980s. There were some evacuations from the Falkland Islands, so if they really want evacuations, there is a way they could do that. Also, Maybe a reason to change the timeline is they wanted King Frank to be through World War II. The third reason I thought of, (I consider it very unlikely) Polly and Digory are going to be kids in the next movies and Digory will not be the Professor at all. They just wanted all the stories to take place in the 50s. 

@eustace The Falkland Islands are a long way from England. They're almost at the south tip of South America. Apart from British military personnel, the population is 2000, all of British descent. I haven't found any evidence that children were taken away from the islands for safety, just a few hundred evacuated to the army camp.