The first question I had when reading the previous chapter (why here?) is answered swiftly by Hugo: he tells us of Valjean that: “like a bird of prey, he had sought out the remotest spot he could find for the building of his nest.”
Upon their arrival, Valjean unlocks the door and then unlocks a second leading to a small apartment. The furnishings were sparse: a small stove, a table with a few chairs, a simple bed, and an inner room with a small trestle-bed for Cosette.
Cosette, still sleeping, is laid into this small bed and remains asleep, with Valjean dropping to his knees in gratitude to be in this place with the child.
The next morning the child is startled awake and immediately begins looking for her broom and assuring mrs. Thenardier that she is about to start cleaning. Upon seeing Valjean she greets him enthusiastically and celebrates that it was all true - it’s as though the poor child had feared everything was a dream.
As the day rolls on the child, possessing nothing but the doll that Valjean had gotten her and the newfound freedom of no longer being a captive of the Thenardiers declares that the place they share is “lovely” and seems genuinely awed by all of it.
Perspective makes a world of difference. What Hugo described to us as a frightening and hellish place we now see to be a wonderful place for a new start for Cosette. She has her freedom. She is loved by Valjean. She has a doll of her own that she never would have imagined. For the young lark, life is beautiful for the first time in her memory.