"The music critic who panned Margaret Truman's singing was Paul Hume, 1915-2001, who married in 1949 and had 4 children: Paul, Michael, Ann, and Peter. I don't remember what Who's Who has for Brit's parents or date of birth, and couldn't find anything useful with a Google search just now; he can't be Paul Hume's grandson but perhaps is related in some other way. I thought Brit was a nickname. I don't know either, but if Paul Hume had children at 20 or 21 as did his children, Brit could be a grandson. I note that people in my generation married and had children much earlier: it was rare to wait till the age of 25 to start your family.
I checked
Who's Who today for Brit Hume (full name: Alexander Britton Hume). He was born June 22, 1943, and his parents were George and Virginia Powell (Minnigerode) Hume. Paul Hume did not marry until 1949 and was only 27 years old when Brit was born.
Truman called Paul Hume "a frustrated old man" at the time of his panning Margaret's singing; at that time Paul Hume was 35 years old and President Truman was 66.