Final answer:
Fantine's daughter in 'Les Misérables' is named Cosette, and she is roughly three to four years old when first mentioned in the novel.
Step-by-step explanation:
Fantine's daughter's name in Victor Hugo's novel Les Misérables is Cosette. When the readers first learn about Cosette in the novel, she is approximately three or four years old. However, her specific age at different parts of the story can vary as the novel covers several years of her life. Cosette is a central character in the narrative, whose life represents the struggles and hardships of the lower classes in 19th-century France, as well as the redeeming power of love and compassion.