Hi, Jeanne! One of the things I really like about the new on-line Advance Dining Reservation system is that it gives you alternate choices if your first choice is unavailable.
So, I went to check on one of your dates. I punched in Magic Kingdom for lunch on August 26. While my first choice (Crystal Palace) was unavailable for lunch, it gave me options of Liberty Tree Tavern, Plaza Restaurant, Tony's Town Square Restaurant, and even Cinderella's Royal Table! For dinner, on that same date, Crystal Palace was available for a late dinner and they also gave an option of California Grill at the Contemporary. (Please note that these same restaurants may no longer have availability when you go to check!)
With this late date, you will have to be flexible, but there are many dining options open. You may not get your first choice, but try something new - it might just become a favorite! When you get to your resort, see the Concierge for any last minute openings if you have a restaurant you really wanted but could not book. Also, consider booking lunch instead of dinner as another option.
Check out WDW restaurants
HERE and start checking again. Take your time, be flexible and I think you will be successful. Good luck!