For those of you who may be struggling with how to discuss death with a little one, I highly recommend the book "The Fall of Freddie the Leaf" by Leo Buscaglia.
I lost my dad when I was 5 (in 1987) and this became one of the most impactful books I read as a child. I kept my original copy for many years and anytime I missed my dad I would go find it and read it... even when I was in my teens.
Even if they haven't personally experienced a loss, it still makes a solid, positive, non-religious introduction to the subject in a way kids can get... I emphasize the non-religious aspect because it allows the text to be incredibly versatile. Each family can add their religious views when discussing the content of the book but it doesn't emphasize one religious view over another.