Get an unsweetened bar of baking chocolate and cover label with a piece of paper and label it "SINS".
Have a good-natured girl volunteer. Then have her take a bite of the "SIN" bar. (Have a garbage can ready for her). Ask her how it tasted. Her response should be quite negative. "Gross, sick, bitter!"
Then have her take a bite of the "REPENTANCE" bar. Ask her how it tasted. Her response should be "smooth, sweet, good".
Tell the girls that the chocolate is like sin and repentance. No one can really tell by looking at the outside what the chocolate is going to taste like. But experience shows how sin is bitter, not something you want. You want to spit it out, like the unsweetened chocolate. The milk chocolate bar is sweet and good. Just like how you will feel after going through the repentance process. You want to savor the milk chocolate, just like being free from sin.