Conversations from the back

Overhead in the car this afternoon on the way home, but first, some definitions.

Mattie= the girl's best friend (she's 3)
Pawpi= my father (he's 60)

The girl: Mattie's big.
The boy: How big is she?
The girl: She's 50!
The boy: She's 50? No way, if she was 50, she'd be dead!
The girl: How old is Pawpi?
The boy: He's 48.
The girl: Mattie is 50! She's tall!