I am sorry for your loss. I bet she was very special and what a great thing your son will always remember her.
He is 8. He is old enough to understand that great grandmothers body was old. That it was tired., but weren't you all lucky to have known her? Maybe talk about things you will always remember about her..
If she was ill or in pain, speak about how now she is not hurting, feeling ill.
Also be truthful about how YOU feel. Do not try to hide your sadness.. It will make him think that is what he is supposed to to do.. Instead ask him for a hug. "I need a hug. I was thinking about Great Gran and since I cannot hug her, I need to hug you."
I want to be cremated. I explained this to our daughter when she was young. I explained the reason is because I did not want to "waste space".
That eventually if everybody were buried in giant plots, that land will not be usable.
I also told her "I do not want you to feel like you need to go and visit this place." instead she can either keep my ashes, or release my ashes into the wind so that I am all around.
Our daughter was also with one of her Great Grandmothers when she took her last breath and died. We felt so honored. Our daughter was not upset, instead she "felt honored". We had explained how GG had specifically chosen for us to be there at that moment that she left her body.. due to the circumstances.. She said she was "not frightened, because it was so peaceful, not dramatic like on TV or movies".