Death is a very difficult thing to talk about to a young child as there is absolutely nothing positive about it. It is a time of personal and irreparable loss and a child will feel it acutely especially if the death has occurred in the family. If the loss is of a beloved grandparent then the child is at a loss to understand what has happened and why that person is no longer visible. A parent has to cope with his own grief and explain to the child about the tragedy.
You should not shy away from the subject but broach it in a gentle manner. Perhaps the child has seen that a grandparent was not well or had to be hospitalised. You could ready him for the eventuality by explaining that when people get old and infirm they cease to be and go to God to be made new and whole. Also the importance of efforts to be healthy by eating right, exercising and taking proper medicine when needed could be highlighted. You could assure the child that he alone is not feeling low but you are also feeling sad and grieving the loss and that this feeling is normal.
You should not try to shield the child too much but discuss it in an open manner. Explain what the process is about and why that person has died. Answer any questions the child may ask and comfort him by telling him that all of you are brave and will cope with the loss.
Often the child is faced with the death of a pet dog or cat. Sometimes the child may not have wanted to help in looking after the animal and may feel that the pet died due to his fault. The parent must dispel this guilty notion and explain that this is the law of nature and death is a natural happening. Humans do not control it and the death is not his doing.
With the death of a loved one, parents also grieve and are not in a state to do much explanation but they will appreciate that it is a new feeling for the child and being unfamiliar with it, the child will find it difficult to come to terms with it. So a little time spent to explain will go a long way in helping the child face life.