A child grieves like an adult but he expresses his grief in different ways. Because of his emotional immaturity, he may not have the thinking abilities to make sense of what has happened. He tends not to have the words to describe his feelings, thoughts and memories. Therefore his behaviour is your guide.
Is he irritable over every little setback? Does he burst into tears every time he is thwarted? It might help to think of his grief as a deep river, flowing beneath the surface of his life and like a river, every now and then the pressure becomes intolerable and bursts through the surface. This behaviour is very hard for you to bear because you probably feel the same way. If you can, hold him, comfort him, just as you would like to be comforted yourself. Gradually this tempestuous behaviour will settle down, although it may still recur from time to time.
(from Grief and Bereavement: Understanding Children by Ann Couldrick)
When a child dies, other children in the family, however young, will realise that something has happened and will need help to express their feelings. It is important to explain to your children why you are sad and upset, in words that they can understand. Some children may think that it is their fault. Answer their questions honestly, however difficult. Help them to understand that you love them and reassure them that the death of their brother or sister does not mean that they will also die.
To understand more about the needs of grieving children and young people you might find the following information sheets useful;