Cain's wife's parents were also Adam and Eve. Scripture records the names of three of Adam and Eve's children (Cain, Abel, and Seth), but they likely had more children whose names were unrecorded. What you can tell your friend's child is, "Until the human race had grown to a population large enough to make such arrangements unnecessary, God allowed brothers and sisters to marry and have families together."
__________________ "If anyone comes to me, I want to lead them to Him." --St. Edith Stein