John Dawson, Barbara Southard and Rosemary Brett were photographed in 1941 huddling together at London's King's Cross station before being put on a train.
The children, each wearing an identity tag, had already endured months of the World War II air attack by Hitler's Luftwaffe, which began in 1940.
Protective Barbara, five, is seen putting her arms around John, three, and Rosemary, two - as they waited to be dispatched to the safety of Northampton.
The trio, now 76, 74 and 73, eventually returned safely to their home in Greenwich, South East London, where John still lives. Barbara, now living in nearby Welling, said: "It brought back lots of memories."
The stamp is one of a collection of 12 WWII designs on sale at 60p for a year to mark the 70th anniversary of the Blitz.
One features wartime PM Winston Churchill inspecting troops. In another, the Queen is seen broadcasting to the nation as a young princess watched by her sister Margaret. Another features an air raid warden setting "lights out" time.