This complex novel has a number of different things to say about parental love. Parental love is used to both show the redemptive power of the action of loving and also the moral injustice of society. Consider the character of Valjean, who achieves his moral regeneration through loving Cosette as his daughter. He is a character who lives his life based on the following moral maxim:
To love or have loved, that is enough. Ask nothing further. There is no other pearl to be found in the dark folds of life.
Parental love, even though Cosette is not actually Valjean's daughter, allows him to demonstrate the truth of this quote in his own life. He becomes a character who is defined by self sacrifice and love for others, and through this, manages to achieve moral supremacy in a world that shuns him and demonstrates a complete lack of grace.
However, at the same time, parental love is used to expose the hollow core of society's morality. This is most clearly shown in the case of Fantine, who is forced to sacrifice her child in order to simply survive in the world in which she lives. Because she is on the lowest rung of society, she is shown to have no rights, and even what should be most sacred--the bond between her and her child--has to be sacrificed in the face of society's profound indifference to her plight.