Anne Frank. Dd2 loved it; she asked if there was a sequel and I had to explain the sudden stop in the book. 
Ds is very interested in WWII so we have a number, although a fair number are factual. Usborne has some good factual ones.
He's got a number out from the library in a set called "War at Home/war at sea/war on the land/war in the air" by Anthony Masters. They are very blunt though, no softening of the facts, they are I think true stories.
On gentler ones, you get the Chalet school ones set during the war: "In exile"-when they have to flee the Nazi's from Austria, "at war" and "Highland twins"-you get things like rationing, air raids, and blackouts mentioned.
Noel Streatfield has a few "Curtain up" is really end/just after the war, but "The Children of Primrose lane" is a spy story during the war and "When the Siren wailed" is a really good war story, again evacuees, but a really well rounded story about children during the war.
We had a book called (I think) Riddleton Roundabout, about 2 children evacuated to the country, just doing normal things for the war effort. Don't expect it's in print though.