In 100 years people will still talk about Spartacus but I doubt anyone will be talking about Romancing the Stone or Wall Street or Falling Down or Fatal Attraction or Basic Instinct. Those were all fine films, but Kirk was the star of movies that defined the evolution of the history of Cinema. He was THE star of his time and it was at a glorious time.
Kirk Douglas is more famous than his kids. If for no other reason, he has been around longer and hence has reached more individuals in that time with his movies. While his son Michael is a close second and is more active in recent memory, it can still easily said that the star such classic movies is more famous.
Douglas has been in many more films and has more recognition that his children due to the fact that his career is longer and he set a path for himself while his children are riding in his limelight. His age simply exemplifies how accomplished he is and his birthday simply pushed his fame further.
Kirk Douglas is an American legend and it is incredible he has made it to the grand old age of 100. But with the media consumption of younger people and the way the world is now more people would probably be able to name Michael Douglas from a photo than they would Kirk.
The baby boomers might remember Michael Douglas more, but Kirk Douglas is actually more famous. Kirk Douglas was famous at a time that there were not many movie starts. That means that the movie stars that did exist were very, very famous. The elder Douglas is an example. He was so handsome that he starred in many things.