their 'firstborn' [='eldest, oldest'] child
Parents often spoil their 'firstborns'.
(..) Well, you don't mind that. You know who taught him. But God has a sense of humor and God said, ''Let him have a girl.'' And l had my child there, firstborn, at home, and it does something to you when you become a father. You're home, you know, really home. And the baby was dirty, (..)1983 Bill Cosby: Himself