their 'firstborn' [='eldest, oldest'] child
Parents often spoil their 'firstborns'.
(..) to get going. That ta boy. So, uh... you're, uh... Santa Claus. Right. Would you mind taking this to your firstborn? Sure. Um, uh, and my firstborn, he's an elf? Yes. Actually, I'm adopted. Uh, Michael, would you open this (..)2003 Elf