In payment for helping break her father out of prison, Gavroche asks Eponine for a kiss. She believes he means a romantic kiss (and says he is too young to kiss a woman), while he clarifies that he wants her to kiss him on the forehead the way his mother did when he was young (which she does). In the novel upon which this is based, Gavroche was Eponine's younger brother, abandoned by their parents to the streets of Paris. That connection is neither confirmed nor denied here, so it's possible that either he was asking for a romantic kiss from his own sister, or he is asserting that he recalls affectionate kisses from a mother who abused and abandoned him.