Oh, my goodness. What a huge question! I was literally out of time last week so I promised I would address the topic today.
I really tried. I had visuals, I had stories, I had comparisons. I thought about it all week--trying to figure out how to explain such large doctrine to such small people. I read and studied, I asked friends and family for input.
I'm pretty sure I bombed. I'm pretty sure that it went right over their heads. I'm pretty sure that when their parents ask what they learned in Primary, they will shrug their shoulders and answer, "I dunno."
I am not always a profitable servant and while the children may have learned nothing, my own understanding of grace grew. One of the points of the lesson that will stick with me was that we are not earning heaven, we are learning heaven*. The price to return has been paid by my Savior, now I am trying to come unto Him, to become like Him; in this way, I hope to someday feel comfortable with Him. His grace has saved me, it is up to me whether I will let His grace change me.