To begin, read the story from Genesis Chapter 4 in the Bible. You may use a picture Bible so long as it tells the story in enough detail that the children are able to understand what motivated Cain, which was jealousy over the favor that God showed to Abel’s sacrifice over his. Try to provide at least pictures representing the two sacrifices they made: Abel’s lamb and Cain’s vegetables.
Beyond reading, it’s important you make sure the children grasp the essential points. First of all, that Cain didn’t give his gift from his heart; he didn’t give the best he had, and he didn’t do it because he really wanted to please God. Abel gave from his heart and that’s why God accepted his sacrifice. Cain’s bad heart showed in how he reacted. Instead of being sorry that he didn’t give God a good sacrifice, he was angry at Abel, who hadn’t done anything wrong, and that’s why he killed him.
Move the talk from the Bible characters to the children. Ask them if they know what it’s like to want something someone else has and if they ever get angry or hurt another because of it. Then talk about how they get afraid when they know they did something wrong, because they don’t want to be punished. Cain felt the same way; he didn’t want God to punish him. When they sin, they have to do the same thing that Cain did, which is ask God for forgiveness and help.
Let your preschoolers color a picture such as this one at “Thus saith the Lord…” Ministries. As they color, talk about how much God loves to have them give to Him and others with a happy heart. Just like Abel gave the best he had, the best gifts are things they love, which they give because they love the person they give them to more. God didn’t reject Cain’s sacrifice because He wanted a lamb more than He wanted vegetables, but because He cared about his heart.