I was about 7 when the reality of death really hit me. I remember saying to my dad 'so you mean when I die I will just be dead and in the ground and no one will remember I was here?' and his answer was basically 'yes'.
I cried and cried - but I got over it.
When I have kids I will gently explain that some people belive that you go to heaven, other people belive that you come back and I believe that you stop breathing and your body stops working but that you remain behind in other people's memory until they die and if you have good friendships then they will tell other people all about you.
I won't be doing the heaven stuff because I don't belive in it.