Are you not yet finished or not?

Mommy: Dominic, Pia, are you not yet finished?

Pia: Yes!

Dominic: No!

Mommy: (confused) Finished or not?

Dominic: Mommy, when you ask 'not' you don't know what we mean when we say yes or no.

In other words, Mommy's way of asking questions lead her to confusion about the answer. Does Yes mean yes, we are not yet finished, or does 'No' mean, no we are not yet finished.