When the Wise Child asks their question, the parent is commanded explain all of the testimonies, statutes, and laws to the Wise Child, up to and including the law dealing with the afikoman. Hence one is to teach the wise child all of the laws of Passover from beginning to end, from the proper preparation of the festival to the conclusion of the Seder night with its final prohibition. 

Another reason for mentioning the particular law of the afikoman is that on a halachic level, it is prohibited to eat anything substantial after the paschal sacrifice or after the afikoman matzah, whose taste must remain in the celebrant's mouth. The Wise Child has asked several intelligent questions. His/her approach to Judaism is intellectual. He/she wants to understand and to reason, and you must instruct him/her. However, you must also teach him/her that Judaism is not merely an intellectual pursuit. While he/she may comprehend the laws cognitively, he/she should also remember the taste of the matzah. It is not enough only to study Judaism; one must also practice it. And the joy of practicing should be as a plasant taste on the tongue remaining long after the actual performance of the mitzvah. 


haggadah Section: -- Four Children
Source: The Passover Haggadah by Rabbi Shlomo Riskin