Developmental stages in a child's understanding of death or loss
Birth to age 3: A child views death as a loss, separation or abandonment. They are less disturbed by losing someone than an older child because of their difficulty understanding the whole concept. The most important element at this stage is the response of the living parent and significant others around them. If that security remains intact and schedules remain as normal as possible, they eventually make it through. They take their clues from the security or lack of it around them. It isn't that they don't grieve and we shouldn't pretend nothing has happened, it's just they gain security and transition based on the living parent's response to grief.
Ages 3 to 6: At this stage a child sees things as reversible and temporary. They may believe in "magical thinking" and that their thoughts can cause things to happen. This can work in either direction causing them to blame themselves unnecessarily or believe if they are "good" enough perhaps their parent will return. Often, children will exhibit nightmares, confusion, revert to an earlier stage of development or even seem to be unaffected by the death.
Ages 7 to 8: Here a child will begin to see death as final. They may have lost an animal at this point but they usually don't think about it as happening to them. They see it more as something that may occur in an accident, like a car accident or only in old age. They may show an unusual interest in knowing the details surrounding death, begin asking what happens after death, or again act as if nothing has happened. Social development is occurring during this stage so they'll watch how others respond and may even want to know how they should act.
Ages 9 and up: By now the child understands that death is final and irreversible. They not only know it could happen to someone else but also to themselves. They may exhibit a wide range of feelings including: shock, denial, anxiety and fear, anger, depression even withdrawal. Their reactions begin to be much more like an adult except they may act out their grief by behavioral changes at home or school.
I also wanted to illustrate a coping technique appropriate for most ages. This tool even works for adults: drawing or writing about feelings. So I've picked a subject matter we all have gone through as a normal stage of development. We've all thought at one point there was a monster under our bed.