Ask someone in the class if they would be willing to hold a string. After they have the one string, ask them to break it. After they break the string, give them two and tell them to try again. Continue to add a string or two until they are unable to break the bundle. As you add strings, ask the person if the bundle of string was easier or more difficult to break than last time. When that person is unable to break the bundle, ask if they have any hope that they can break it themselves. When they admit they don't have hope. Ask if they would like to ask for someone to help. Give the helper some scissors. After their "helper" has cut through the bundle of strings. Ask both of them how they feel. Thank them for their participation.
Then, draw one string out and explain that this is a sin. (If there is a specific type of sin this age group or class may struggle with or the lesson focuses on name that sin). Explain that with one sin, such is easy to repent of, but as the sin is committed again and again it becomes more difficult to break - it becomes a habit. Repentance of the sin becomes more difficult because the desires of our heart are more on sin than they are on choosing the right.
At some point, we cannot break the habits of sin without the help of our Savior. We become hopeless. If we turn to him, with a broken heart and a contrite spirit, and repent, he will help us break the habits of sin. He will cut the invisible ties that bind us to the adversary. His atoning sacrifice will bring us hope.
Matt 12:29, Matt 23:4, Mark 3:27