Here is an illustration I use with older children to help them understand what Jesus did for them on the cross and the need for their response.
Ask a child, “What is your favorite store to shop in?” (They respond.) “What if I were to give you a gift certicificate for $100 for that store? You would go to that store, pick out $100 worth of things, bring them to the counter, give them your gift certificate and then pay the $100, right? Of course not! The $100 has already been paid – you just give them the gift certificate knowing that it is good enough to pay for all that stuff.”
“But if you just kept the gift certificate in your pocket and never used it, it wouldn’t do you any good, would it? When Jesus died, He took the punishment for every sin. But you need to ‘cash it in,’ by believing. (Through faith.) To not believe would be foolish, because Jesus not only paid to make a way for you to be saved, He paid to make the only way.”
Be creative. Perhaps to remind them of this truth, you could give out gift certificates to one of the local fast food restaurants. As they approach the counter, they will certainly be thinking of this precious truth.