From left, Billy Gray, Jane Wyatt, Robert Young, Lauren Chapin and Elinor Donahue look at a newspaper on an episode of “Father Knows Best” on May 16, 1956. Chapin, an actress who played the youngest of the three wholesome, upbeat, all-American children on the popular 1950s sitcom but whose personal life was a traumatic contrast to her best-remembered role, died Feb. 24. She was 80.