This symbolic child makes an appearance during the 'four questions' segment of the Hagaddah, the book that accompanies a Pesach Seder and guides us along the path of ritual. Each of the four questions is represented by a different child in the story and each section is traditionally read aloud by children at the Seder table.
One child is arrogant, one is wise, one is young and unaware, and the last cannot even ask questions because he is too young to even speak or doen't have sense enough to think to ask.
To the fourth child, we are instructed to kindly and gently describe the Jews' plight in Egypt, what it means for all Jews today and the significance of our recognition of Pesach (Passover).