Thank you for this excellent question. There is nothing so precious as seeing a child grow in interest about their own soul and their need for salvation. This is result of growing up in a home environment that truly exalts the Lord and the glorious gospel of Christ. I commend you for putting first things first with your child.
However, dealing with the soul of your own child is also a frightful thing. The dangers of "messing this one up" are eternal dangers and I understand your concern. It is also amazingly difficult to understand just how much the child understands and how much they are simply mimicking what we want them to say and do. They want so much to please at this age and we can so easily lead them into saying just about anything. But salvation is of the heart and obedient repetition of words do not save. I have likened such dealings to spiritual surgery. The word of God is the scalpel and malpractice does not lead only to death; but to eternal death. We have good reason to tread on such holy ground with our shoes and hats off in silence.
But while healthy fear is good and may keep us from doing really harmful things, we must not let this fear freeze us into doing nothing. God has called us to the ministry of reconciliation and His Spirit will guide us if we will trust in Him. I do not have any perfect answers to this one, but I would like to make some observations and suggestions that I pray will help you in dealing with your son.
A young child can be saved, but they too often go through the motions to please parents or other authority figures or to mimic what they have seen others (like an older sibling) do. One of the majors checks against this is to make sure that the child understands his or her guilt in sinning and knows that this is the problem that must be solved for entrance into heaven. You are heading in the right direction on this issue. I fear many today totally avoid this to the child's harm--the main harm being an assumption of salvation by one who never got saved. No one can be saved unless they first realize that they are lost.
Therefore, when I speak with young children about salvation, I always make sure that they understand sin and their guilt. I look for some level of conviction (understanding that this conviction will be displayed much differently in a child than in a hardened adult). If I do not see a true understanding of sin or of their personal guilt, I will often put them off for a while. However, this must be done carefully. It is good to tell them why they are not ready and then to pray with them specifically asking God to preserve them during this time and to give them fuller understanding. This should be followed up by parents or others every so often to see where the child is.
Yet, we also need to be careful not to put the child off too much. They should not be frustrated but encouraged to be saved. We must let them know that the Lord will continue to help them so that they can be saved soon. However, we should also not be too quick to put them off. They do not need the understanding of an adult. The main thing is that they have a true (though simple) understanding of what salvation is. With some children, it is difficult to tell exactly where they are. They tell you what sin is and admit their guilt as a sinner, but you are unsure if it is real understanding or a repetition of what others have told them. With these, I generally go ahead and lead them into salvation.
However, I am very up-front with them. I tell them that I think they understand and that they are probably ready to be saved, but I tell them that if they worry about it at some time in the future and need to make sure of their salvation, that that will be alright. Then, when I pray for them after they have called on the Lord for salvation, I specifically ask the Lord to preserve them if they do not fully understand and that, if necessary, He would bring them to Himself at a later date. I can see some people accusing me of bringing doubt at a time when we are to encourage their faith. But I have not found this to be the case. I present this information in the most positive of terms and I have found children to be eternally optimistic in these areas. Then, with these children, I encourage the parents not to push immediate baptism. If they are really saved, they will want to go ahead and be baptized. But this gives them time to make certain a bit more. This does not solve all the problems in dealing with the souls of children, but in my experience it has brought about more certain conversions in the children. I realize that some people would criticize me for not being aggressive enough, but I have seen way too many false conversions give people a false hope that in turn leads to destruction. I also trust that the Lord can and will preserve those who are looking for Him.
You asked for illustrations for explaining how Christ paid for our sins.
Consider this: "What if you were in a store and accidentally knocked over a mirror and broke it. Then, the store owner came out really mad because you broke it. He told you that this mirror was worth a hundred dollars and that if you did not pay for it, that you would have to go to jail.
But you do not have a hundred dollars because that is a lot of money.
Then, a man walked up--a man you did not know--and he said to the store owner, 'I will pay the hundred dollars so this little boy will not have to go to jail. All he has to do is ask for my help.' How would that make you feel? Well, that is like salvation. You have sinned and your sin has to be paid for by you being punished forever. But God sent His own Son to come to earth and pay for your sin so you will not have to be punished forever. All you have to do is to trust in Him and ask Him to save you.What do you think about this?"
This kind of a story could be told in many ways depending on the understanding of the child. As much as possible, use specific things because this is the way a child thinks. You might use "Wal-Mart" instead of "a store" and refer to something in that store and the policeman that watches over that store. I am sure you get the idea. You can also illustrate it by punishment. "What if you did something very bad (mention something they would understand) and were going to be punished for it? Then, what if your someone came and said that they would take your punishment for you is you would ask them? This is like salvation too."
I hope these suggestions help. Always remember to pray through as you talk to children about their salvation. It is a high and holy business to deal with souls but God's grace is sufficient for this as well. God bless.