John recognized Jesus with two titles: "Lamb of God" and "Son of God." The first title described Jesus' ministry while the second described his relationship with God the Father. "Son of God" told us who Jesus was. "Lamb of God" told us what Jesus did.
Lambs are mild animals with many uses. The gospel writer John wanted to use these characteristics to flesh out his view of Jesus. The mild, innocent man led to slaughter. Indeed, unlike Matthew, Mark, and Luke who placed the death of Jesus on Passover, John placed his death on the day before. John had Jesus die on "Preparation Day" when the lambs were slaughtered for the Passover feast.
Do you have a stuffed lamb at your house? Use it to describe the uses of lamb (wool and meat come to mind). Have your family members draw lambs and glue on a "wool" (i.e., Dacron fibers from pillow stuffing or other such materials). Have your family members draw a cross in the background to make connection between the humble animal and Christ's sacrifice on the cross.