The Silver Sword by Ian Serraillier

The story begins in 1940 when Joseph Balicki, the headmaster of a school in Warsaw, is arrested by German soldiers for disrespecting a portrait of Hitler. Shortly after Joseph is taken, German soldiers break into the family house and take his Swiss wife, Margrit, to Germany as a forced laborer. Their 11-year-old son Edek shoots at the soldiers in a bid to save his mother and narrowly escapes the house with his 12-year-old sister Ruth and three-year-old sister Bronia before it is blown up by the soldiers. The three children begin living in a bombed-out house and Ruth starts a school for children living nearby. Edek becomes involved in the black market, smuggling food and clothes for his sisters and children living with them, and is captured by Germans searching for hoarded goods.