It is very difficult for many children to cope with the death of a loved one. As a parent, you need to understand that each child reacts to loss in a unique way. It's important for you to do your best to understand your child's reaction. Answer any questions your child has in a simple, honest way. If it's possible, prepare your child in advance for the upcoming death.
Don't compare death to sleep, when you try to explain it to your children. This will confuse them, and make them think that it's possible that the person they lost will one day awaken. Worse, it could make them confuse sleeping with death.
Be honest with your children without losing control of your own emotions. Make use of any religious supports. Many kinds of religious leaders offer guidance during times of loss.