Hi Laura!
Firstly regarding The Garden Grill restaurant - keep checking
online to see if anything becomes available - cancellations do occur all the time, and you might just find a time to suit you. The advantage of checking online is that it is really quick and easy to do.
If you are looking at other character dining, I would recommend
Chef Mickey's at Disney's Contemporary Resort. This is a lively, fun experience that everyone will enjoy - and you meet Mickey, Minnie, Goofy, Pluto and Donald.
For meeting the Cinderella and the Disney Princesses you would love Cinderella's Royal Table, located in Cinderella Castle, in the Magic Kingdom park. You will meet Cinderella for photos with your family before being shown to your table in the dining room, then some of the other Disney Princesses will visit with your table. As this is very popular, it may be a challenge to secure a reservation for September, but again, keep checking. Another opportunity to dine with the Disney Princesses is at Akershus Royal Banquet Hall in the Norway Pavilion at Epcot.
I would also recommend
The Crystal Palace in the Magic Kingdom park. This is in a beautiful conservatory setting, and you will meet Pooh & Friends.
As I am limited for space here, please send me a direct question with details of your family's dining preferences, and I will be happy to help you with some more dining ideas.