I have found this illustration to be equally effective with 5 years olds all the way up to adults. It is especially useful for those who don’t understand the function of Jesus.
Place a long piece of paper on the floor (I used the paper from one of those big rolls). Cover the paper with flour. Ask for volunteers (or have everyone do it) to take off their shoes and socks. Tell them to walk across the paper (they can’t jump) without getting flour on their feet (this is obviously impossible). Ask them how they think they can get across without getting flour on their feet. Eventually, place a couple chairs or a bench across the paper. Have everyone walk across the bench. They will successfully reach the other side without any flour on them.
Ask if anyone knows what the flour represents. Explain that it represents sin, and that we cannot avoid it as we walk in the world. Explain the significance of the bench (which represents Jesus). The bench is our bridge to the other side of the paper, just like Jesus is our bridge to God/Heaven.