My husband and I tried to address his questions at the dinner table that night and soon just realized that somehow our verbal explanations were simply not making sense to him. So we asked him if he would help us figure out some of the answers the next night at Family Home Evening. That was definitely a good suggestion and was met with much cheering and speculation about what the treat would be. Family Home Evening (or perhaps it is the treats that accompany it) are definitely a hit in their minds.
That night I had prepared various cut-outs from the Primary 6 lesson book. Chapter 1 is about Heavenly Father’s Plan for Us. I had the boys explain to us what each part of the Plan of Salvation and had them help me place it on a piece of paper in the appropriate order. Unfortunately our poster was much too big to scan so I was thrilled when the boys brought home the below graphic from Primary yesterday. Our poster looked quite similar with the exception of less professional coloring and not nearly as fancy.

As we were completing the poster we had some great discussion about what each stage of life was like, what we did in each stage, and how wonderful it was for us to know what came next. Mr. Engineer was quite impressed that the whole plan made sense and kept telling us, "I already KNOW that!" We told them that in its most basic form this plan was the gospel.
Then we covered up most of the poster so it looked like the picture below....

...and asked them how they would feel if all they knew about was that they were born, would live on Earth and then Die. Everyone agreed that it would be very sad and scary too. The boys were quite concerned when we told them that there were many people on earth that only knew this part of the plan. They didn't know that they would be together with their families after they died and didn't even know where they would be after they died. That the missionaries job was so important that they went and found people that didn't know the whole plan and shared with them that we lived with Father in Heaven in the Premortal world before we were born and that if we lived righteously we could live with Him again and our families after we died.
I thought that our lesson was pretty much concluded when Engineer boy asked what would happen to people who didn't learn the gospel while they were on earth. Obviously we had glossed over the role of the Spirit World a little too much. So we explained about others being able to learn about the gospel while they were in the Spirit World and have members be baptized for them in the temple.
When we concluded both boys wanted to be Missionaries so that people wouldn't have to be sad or scared about what would happen to them after their life on Earth.
I know I haven't done the experience justice here, but it was truly a wonderful night with the Spirit present in our home.