When Jean Valjean first visits Cosette in the Thénardier house, he is shocked to find what a terrible life Cosette has been living. One of the first things he does is give her an expensive doll as a present. But Cosette is so used to terrible treatment that the doll takes on an almost religious significance: "Cosette gazed at the miraculous doll with a kind of terror. Her face was still wet with tears, but her eyes, like the sky at dawn, were beginning to glow with a strange new brightness" (220.127.116.11).
The doll in this case symbolizes that transformation that's about to take place in Cosette's life. It's like she's seeing light for the first time, and it's so foreign she's afraid of it. How depressing.