symbolically, what does Daisy's little girl represent

Gatsby has been romanticizing Daisy. This romantic vision does not include a child. Having a child means Daisy is human and Gatsby prefers his illusion of Daisy rather than the woman herself. The child also reflects just how irresponsible and self-serving Daisy is. She never cares for her daughter, preferring instead to go to parties and live the life of a young socialite.