The one thing I would encourage is that you try to avoid comparing it to sleeping. Lots of children become quite disturbed at the idea of going to sleep if they think there is a possiblity they mightn't wake up.
I completely agree... I was one of the kids that got too scared to go to sleep in case I didn't wake up.
My older kids learned about death through the passing of one of their pets. It made it easier for them to understand when their Grandma passed away.
We have also tried to explain that it isn't just old people that pass away, it can also be people who are very sick (because they have known of children to pass away) or by an accident of some sort... we have also reassured them that as their parents, it is our job to keep them safe from avoidable accidents and to take them to the Dr to make sure they don't get too sick.