You’re
The worst thing that can happen when you are in an argument with someone is that you correct their grammar. That just means that you either have lost the fight and have nothing else to say, or that you really picked a bad time for your English lesson. This mother just used it as a comeback.
The little girl did not see that one coming and basically got destroyed by her own mother who has years of experience ahead of her. So we guess that in the end, she was just no match for someone who had a few decades of a head start on her life.