Conversation:

Joseph: (this wasn't so much a question, but a statement of truth that he was looking for corroboration on)"Mommies never die. Right, mommy?"
Mommy: (that came out of left field and I had to think about what to say for a minute or so) "Well, hopefully mommies won't die till their kids are all grown-up with their own families. You see, dying is part of Heavenly Father's plan. He is your daddy and my daddy and he wants us to come home to him one day."
Joseph: "How does he do that?"
Mommy: "Well we get back to him when we die. That is why we have to be very good and do what is right in this life, then we can live with him."
Joseph: "How does he take us back to the planet?"
Mommy: "I don't really know, only he knows. But you know what?"
Joseph: "What?"
Mommy: "Heavenly Father has promised us that we can live with our families forever all the time. We just need to be good and we can see our mommies and daddies and families again when we die."

I don't know if this is exactly what he wanted to know when he asked his initial question, but I didn't want to lie to him. I also didn't want him to be sad. He hasn't brought it up since our conversation, so I don't know if he has thought about what I said. The spirit was present, so hopefully he felt peace in his little heart.