I would say that it is difficult to explain as one does not return from it, but that the analogy of a permanent state of rest is the best possible one, with the added bit that you only dream for a couple of minutes before expiration.
Additional information to take into account would be that one does not feel anything while dead, and thus does not sense the passage of time.
Of course with simpler words and added reassurances that death is unlikely to come to him/her until they are ready for it, which wont be for about a century