Ask them how they would explain what happened to a friend. Share your own feelings , if appropriate. Don't cover up. Help children to see they are not strange because of their feelings. How they are asked will influence their ability to respond.
Children remember death for a long time and from a young age.
There may be other deaths they need to talk about. Assume their power of observation is astute. Initiate the topic of death occasionally. Don't expect the child to initiate. Assume that any out of character behaviour from a child following a death may be related to the death experience.