Vincent Winter
Vincent Winter, who played Hughie, went on to become an assistant director and producer. "The Three Lives Of Thomasina" was his last film as an actor - he was about 16 when the film was shot. He had success as a child actor in the 50s and early 60s, earning a special Academy Award for his debut in "The Little Kidnappers" (1953), and he had no prior acting experience. He also appeared in two other Disney films, "Greyfriar's Bobby" (1961) and "The Horse Without A Head" (1963) with Pamela Franklin.
As self-righteous as Hughie is, Vincent did a great job portraying him and I think he was cute.
He was a Scot - born and raised.
He died in 1998 at age 50 of a heart attack.
Here's some more information about his life and death:
http://www.independent.co.uk/arts-entertainment/obituaries-vincent-win ter-1186802.html
http://almostangels.org/video/html2.htm