When the Siren Wailed by Noel Streatfield

From the Noel Streatfield website:
It is 1939, and the Clark family lives in South London. Although they are poor - and often hungry - they have a happy home. There is talk that there may be a war with the Germans, and one Saturday morning the children are taken to the Town Hall to be given gas masks, which they must carry with them at all times. Then evacuation rehearsals begin: the war is probably going to happen, and when it does, all children in danger areas like London are to be sent to the country.
The evacuation takes place on September 1st, 1939. Laura, Andy and Tim are taken to a village called Charnbury in Dorset. They are billeted with Colonel Launcelot Stranger Stranger and his servants, Mr. and Mrs. Elk. Here, a new life begins for the Clark children.