Dear English Friends :
Just listening to Lady Mary Soames (nee Churchill) on BookTV talking about her book "Winston & Clementine : The Personal Letters of the Churchills" Houghton Mifflin publishers.
She was asked about her pre-WWII childhood and she said she was "the Benjamin" child.

Can you help me understand what she meant?

A charming lady, very good speaker. While she signed the books (she is left-handed) the camera ran and the the book appears to be about four inches thick!