Talking with children about death and grief is up there with some of the most difficult conversations parents are confronted with.
Here is some guidance:
Be Honest. Explain simply and truthfully what has happened in words the age can understand. Express how you are feeling. Name your feelings. It's ok to say "I don't know" if you don't have the information.
Don't Use Metaphors. Metaphors and euphemisms are very confusing for children. Use the words "dead" and "died." Real words for real things.
Communicate. Talk about how you feel. Encourage the child to ask questions and talk about their feelings. Answer the questions they ask, those are best indicator of what they understand and want to know.
Console. Sometimes kids don't have the language to express their feelings. They may even blame themselves for some reason. Make sure they know that they don't need to hide their feelings, there are no "wrong" feelings to have.
Talk about the future. Children want to know what happens next. Tangible, practical plans give order to the chaos that their lives are thrown into with a loss. Daily schedules, who is picking them up from school, who is making dinner. Giving structure and managing expectations and practicalities will help them feel secure during otherwise tumultuous times.