play dough, small pieces of trash (e.g., old buttons, small twig, paperclip, etc.)
Show your kids some play dough, and explain that you are going to make a heart to represent their hearts before God. As you form it, fold in small pieces of trash. (The kids should respond that it looks ugly, gross, etc.)
Explain that sin does dirty up our hearts and can separate us from a right relationship with God. Now, work on the heart again, this time taking each piece of trash out of the heart until it’s clean. Explain that when we ask God to clean our hearts, He will take away all our sins.