The year 1940 and those that followed were troubled times. The Nazis were attacking Europe, and World War II had started. Wartime changed the clothes that individuals wore as women started wearing trousers and men were fighting in the war. Fashion trends in Europe changed slowly due to the fighting, but clothes for children in the United States changed rapidly during 1940 and the years that followed.
According to the Historical Boys Clothing website, the 1940s was a time of change for boys trousers. The knickers of the 1930s started to disappear and were replaced with jeans instead. Though the trend started in 1940, knickers were still widely worn by boys during the era as well. By the mid-to-late 1940s, knickers were no longer worn.
T-shirts started to take hold during the war to eliminate the appearance of children wearing uniforms. The Historical Boys Clothing website states that the "Hitler Youth" made boys wearing uniforms unpopular and instead casual clothing emerged.
With the rationing during the 1940s, many children's clothes were made at home by cutting down old clothes from their parents. Girls wore short, plain dresses that were often made from home. These short dresses were about knee length.
Boys in school often wore shorts with a plain button-up shirt. Sometimes they wore a jacket and tie, but the formal styles started to fall out of fashion during the 1940s, so the lack of a tie and jacket was not uncommon. The nice button up shirt was a "school" outfit rather than a casual outfit.