Welcome back to SymbolicCharacterNamesVille. Population: Most of the cast of Ragtime.
Like Little Boy, Little Girl represents the future (and the hope for the future). There's a magnetism to Little Girl that is similar to Evelyn Nesbit's: she is attractive both because she's beautiful and because she's lived in squalor. She's been through a lot in her life with Tateh, and she's tough.
So what is the purpose of having Little Girl be a hybrid Evelyn Nesbit/Little Boy? Well, Little Girl is showing us the possibility of three trammeled subgroups of the population: women, immigrants and the working poor. Her ascent from grubby street kid to sunny California girl shows us the possibility of growth and progress.
After all, if Little Girl—who at one point supports her family with her sewing instead of being sent to school—can grow up into an affluent and well-adjusted young woman, who can't?
The early 20th century wasn't called the Progressive Era for nothing: the dream of social progress is what Little Girl experiences as reality. From the tenements to the beaches of the Golden State ain't a bad progression, in our humble opinions.