their 'firstborn' [='eldest, oldest'] child
Parents often spoil their 'firstborns'.
(..) I limp. And it was he... ...that poisoned me. My evil Uncle Imp. Calm down, dear. My firstborn son. My golden lion foully, falsely slain. I pray you found a lasting peace free from strife and pain. Hear my prayer, O cruel gods. (..)2016 TvShow: Game of Thrones Title: Blood of My Blood Season: 6 Episode 6
(..) It's been in our family for years. It's Valyrian steel. Only a handful of them left in the world. It's supposed to go to my firstborn son after I die. To him. He will never wield that sword. If he were to become Lord Tarly of Horn Hill, it would be the end of this house. (..)2016 TvShow: Game of Thrones Title: Blood of My Blood Season: 6 Episode 6