their 'firstborn' [='eldest, oldest'] child
Parents often spoil their 'firstborns'.
(..) couId make it for him. Not l--though many people thought l did-- or anybody else. PhiIIipe knew that my firstborn... his brother Pierre, wanted to abdicate... which he did, eventuaIIy, to join the church. Your father reaIized... (..)2001 The Princess Diaries