“Hideous Kinky” by Esther Freud (post 2): Why does five-year-old, nameless, first-person narrator have vocabulary and sentences of eloquent adult?

I don’t understand how Esther Freud could have intended to write a novel with the following two impossibilities: a 5-year-old child whose mother and older sister never address her by name, and the 5-year-old child narrates the novel using the language of an often eloquent adult.

I still have 30% of the novel left to read, and maybe I’ll figure it out.

1. Esther Freud. Hideous Kinky. New York, Harcourt Brace Jovanovich, 1992.