When children are grieving it is important to get them into therapy as soon as possible. Children don't process grief and loss the same way as adults. From ages 2-8 I often suggest play therapy. Often children externalize grief and loss through play. This allows them to express what is going on in their world subconsciously.
Here is a list of some great books for children who are experiencing grief and loss.
The Invisible String is a very sweet book that reminds children how much they are connected to their loved ones even after they are gone.
The Fall of Freddie the Falling Leaf is a beautiful story about Life.
Lifetimes is a beautiful way to explain death to children.
Gentle Willow, is a story about death, its a beautiful story.