Boys and girls - Scouts and Guides - helped with war work such as running messages, or harvesting crops. Young lads served in the navy and mercantile marine, and many patriotic underage teenage lads wangled their way onto active service. My book Tracing Your Ancestor's Childhood has lots of info on how to find out about records for your ancestors' wartime service and in youth organizations, and my latest feature for Family Tree looks at the effect of both world wars on children, especially at Christmastime.
Images of women bus and tram conductors, and a mother and child hauling a canal boat along the Regent's Canal, from the Nigel Wilkes Collection: The Times History of the War, Vol. IV, 1915.