I've been thinking this up in response to your question, have not tried it yet and it could most likely be modified and improved on. You might have to simplify things for the 6-10 age group. You'll need large sticky labels, about 2x4" or so.
In order to teach the children about imputation you could ask them questions along the lines Ray Comfort uses in the Way of the Master (i.e. Who of you has ever told a lie? How many of you have ever taken something that didn't belong to you?What do you call someone who steals?) Have them write labels as they respond with yes, reading "LIAR, "THIEF", etc. or come up to receive them. Then let them stick the labels on themselves. Tell them, Bible in hand, it is God who labels these actions as sin. You can include other than the ten commandments and leave some out maybe, such as adultery, considering the age group. Ideally they would be fairly well plastered with labels when done(explain this all gently so children won't feel so bad they start to cry, but also don't let them make a joke of it, a tricky balance, prayer needed!). Every adult person present needs to have labels on them too.
You could have them mingle for a bit and greet each other, then ask them how they felt when they read the labels on the other children. Would it be easy to trust them, tell a secret, etc? Then, while they sit with their labels still on, you could have a short lesson on the holiness of God, His absolute purity (perhaps use pure and dirty water as a visual), then ask anyone who wishes, to stand and explain why they think they could be in heaven with God whose eyes are so pure He can not look at sin. Remind them of their labels if they answer confidently, but perhaps no one will stand.... Tell them they can pass on their labels to another person that doesn't have any whatsoever, if they can find one (no one who is present, explain that no other person is without either, though their labels can't be seen, they are there). What a dilemma! What can they do? (Could one include that they are going to hell if the labels/sins remain on them? You might get charged with child abuse if they tell their parent or teacher )
You need to bring out a large cross now or a crucifixion image.You could elaborate on this as you wish, such as dragging in a huge cross and nailing a lifesized carboard silhouette to it or have someone paint a partial crucifixion scene where the face can't be seen. Now introduce they idea that only God is completely without sin, so because He loves us too much to leave us as we are, He sent His son, Jesus, to live a sinless live (the only person ever without any labels) and through going willingly to the cross Jesus declared His willingness to take ALL our sins (labels) on Himself. He alone was able to do this for us so that we could be with God, who is holy and sinless, in heaven forever. Ask if they want to get rid of their labels by placing them on Jesus and have them do so ( stick them to the cross or Christ's hands and feet and shoulders, they don't need to be readable so labels can go on top of other labels). With a teen group one could mention the blood and how the blood of animal sacrifices wasn't enough, as the labels still remained on the person himself.
Point out that they used to have the labels stuck on them and Jesus had none, now He has them all and they have none. Tell them Christ took God's fierce anger over all these sins on Himself and took the just punishment, death, in their place before He rose again. That He is alive and in heaven now. Tell them if they believe this in their hearts and declare (in prayer and to others) that Jesus is the Lord, their Master who died for them but is alive forever, their sins are placed on Him and when God looks at them He sees the sinlessness of His Son and receives them as His own children. Model to them how to pray for forgiveness and thank Jesus for what He has done for us. Sing songs of praise and worship with them to end this such as Amazing Grace, In Christ Alone or similar ones. I think it should be followed by a lesson on prayer the next day.
I don't know, what do you think? I believe this might work with "churched" children but some who have only ever heard today's self-esteem messages might react with shock to being "labelled" a sinner (which would be good, but again, explain yourself to parents). Just some thoughts, as I too struggle to find better teaching material than what is commonly offered for children's ministry and to overcome cultural barriers to telling plain biblical truth.