How do the thenardiers treat cosette?

Worse than an animal.

Cosette is forced to do all the menial chores in the household. She is up before dawn every morning. Her only dress is a ragged cast off from the Thenardier's children. She has never been given a gift, not even on Christmas. Mme. Thenardier finds pleasure in physically abusing her and does so with the slightest inclination. Cosette is described with chilblains on her hands, lips blue with cold, and jutting bones. It is no wonder that "her whole person...expressed and uttered a single idea: fear." (Cosette, Book III, Ch. VIII)