Sweetie is a really spiritual and thoughtful kid. He believes in karma, reincarnation (though he doesn't know that's what it's called), and magic. He believes in God. He believes in the Golden Rule. He loves animals. Over the years I've learned that when someone tells him something or something happens, he ruminates for a couple of days, sometimes weeks. If he has questions, they will pop up randomly some time after the fact, usually while we are riding in the car.
A couple of years ago, Mama G and I had our estate plans done. I needed to get mine done because I am a single mother with young children and needed to make sure they are cared for should their father predecease me, and I wanted to designate someone to handle their inheritance until they reach a certain age. Mama G was most interested in granting a financial power of attorney to someone (not me) and a healthcare directive to control what would happen to her should she become sick or injured and incompetent to make her own medical decisions. As part of this discussion, she told me very clearly that she wants to be cremated.
Sweetie must have heard us. About two weeks later, while we were driving in the car, Sweetie said, "Mommy, what's cremated mean?" He was about 4 years old. I am one of those parents who tries to be honest with my children even when it's uncomfortable, so I told him the truth in as simple terms as I could manage. I said that when some people die they want their body to be buried and other people want it to be cremated, which means that it is burned. Unsatisfied, he asked me how the body is burned, so I told him there are special places that do it and they have really big ovens that they use. (Gasp!) That seemed to satisfy him. About a week later, again in the car and again out of nowhere, Sweetie asked "Mommy, are we going to cook Gigi?"
I think he understands the concept of cremation.