I'm here. :)
My personal belief is that the energy which makes up our consciousness simply gets recycled in the universe after we die. Energy never disappears, it just changes form. So the energy in me today might fuel the growth of a blade of grass later, power a windmill, etc. DS1 would have been intrigued by that sort of discussion. Even at six, he cared for answers more than comfort.
DS2 would have wanted more reassurance along the lines of what you told your child. When I think of stuff like this, I end up thinking about that line near the end of the movie Titanic, when the old lady is talking about Jack and says something like, "He only exists in my memory." I have been known to put a spin on that mentality when talking to my kids about death.
I think you did fine.