My own parents have been gone for years and I also lost my grandparents early in life. It is true that you will lose your family eventually. In my case I lost a son when he was but a young teen. All family is precious.
What struck me most about about what you wrote, though, was how it was so unfair to that little girl to be lied to. You can't grieve for someone you think is coming back. There are some things that can be explained about death to a child that will allow them to grieve.