Dr. Phil advises that children should not be burdened with adult issues. I have been reluctant to tell my son about what is happening, because I don't want him to be burdened.
Instead, we have spent a lot of effort teaching him "foundation principles" that will hopefully help him when the time comes to apply that information. We have had family discussions on the Plan of Salvation. We have talked about the Atonement of Jesus Christ. We have also talked about the temple ordinances that have sealed our family together for "time and all eternity", so that our family relationships will continue beyond the grave. We had a little activity where we used finger puppets (puppet represented the body, finger represented the spirit), and we all had a turn to leave heaven to be born and come to earth and receive a body (finger puppet). Then we talked about how we will all have a turn to die and leave our body behind, and have our spirits live in heaven. And after we all have a turn to die, we will be resurrected (finger puppet back on the finger) and live together as a family. This was made possible because of Jesus Christ and his restored gospel on the earth.
However, the time came when I felt strongly impressed to talk to Jacob about what was happening. I didn't want him to overhear things that might worry him. I figured the best defense was a good offense. So I sat him down and told him I had something very important to tell him. I said that I was very sick. I had something in my brain, called cancer. I explained that sometimes when people get sick with cancer, they die. I told him that because I am sick, my body might die. But I don't want to die. So I am trying to get better. I said that the doctors are trying to help me. And I said that we are praying for help so that I can get better. I told him that if we all pray for me to get better, and if it is not my turn to die, then Heavenly Father will make me better. But if it is my turn to die, then someday I will die and my body will be buried in the ground and my spirit will live in heaven. And I will miss him and Daddy and Emma so much. But after a long time - after everyone has their turn to die - we will be together again. I will always be his mommy. I told him that I didn't want him to be worried or scared, but I hoped he would remember me in his prayers.
I also reminded him of the story we read recently, called "The Kissing Hand". It is a cute story about a raccoon who is getting ready to leave for school, but he is worried about missing his mommy. So his mommy kisses his hand and tells him that any time he misses her, he can use his "kissing hand" to put more kisses on his cheek and feel her love. My son and I had fun giving each other "kissing hands" in case we ever missed each other.
My son took this in stride. Much easier than he did when we told him about my trip to the hospital. That time he cried. Much easier than when his goldfish died. He grieved considerably over that. This time he didn't cry. My biggest concern is sparing him the pain of grief, if the time ever comes to grieve me. Hopefully these discussions will prepare him without causing him to worry.