Children will read you body language and will know if you are upset, so you need to explain. Remember that children need realistic reassurance. Most people do die when they are old, but young people and even children die of serious illness or in an accident. Children are often very curious about ritual and may ask lots of questions about funerals. There are often special prayers and songs.
Some people cry.
There are lots of books which deal with this subject. Some are stories, some are factual. Published by Collins Picture Lions. The Original Velveteen Rabbit , by Margery Williams. Published by Mammoth.