A Dialogue with a 4-(almost-5)-Year-Old on Personhood

(The first few lines slightly paraphrased; I am being invited to play pretend)

C: You pretend to be one of the babies.
Me: No, you can be both babies.
[C gives me a funny look.]
Me: Can't you be both babies?
C: No.
Me: Why can't you be two different babies at the same time?
C: Because I only have one head.

I think she's ready for Reasons and Persons.