Each time a sin is mentioned have them peel off the paper and place a piece of black felt somewhere on their clothing. Tell them the black felt represents dirt. When they have 4-5 black spots on their clothing, ask them if they would want to go out somewhere covered with dirty spots.
Mommies clean dirty spots off our clothes with soap and detergent in the washing machine.
Jesus knows we have sins in our lives that are like dirty spots on clothes. We can try to get rid of our sins by ourselves, but more will come. That is why He gives us something different to wear that covers up our sins. It is called His life of goodness. Because Jesus never sinned and He died for our sins, He wears clothes that are perfect and never have dirty spots on them.