This seems to have been more common in Britain than in the United States. Usually it was boys from wealthy families. Prince Charles , for example, appeared in dresses during the mid-1950s. Usually it was boys from wealthy families. This English boy and his mother were photographed in 1946.
Some period movies depict boys in long hair and dresses.
Movies of course can vary greatly in historical accuracy. It is likely, however, that those who have gone to the trouble of depicting boys in dresses and long hair, probably exercised considerable attention to detail.