Hi Ann,
Celebrating your granddaughter's birthday with a large group at Walt Disney World sounds magical!. Be sure to pick up a Happy Birthday button for her to wear on her birthday. These are available at Guest Relations at the Disney theme parks as well as many other locations around Walt Disney World. They are free of charge, and while they do not guarantee anything extra, you never know when a little extra pixie dust will be sprinkled on her!
Disney's Polynesian Village Resort is a lovely resort. The location is fantastic, with quick access by boat or monorail to both Magic Kingdom and Epcot. There are great dining options including a character breakfast at 'Ohana. Rooms at Disney's Polynesian Resort typically accommodate up to five adults, and you can have one additional child under three years old, so your granddaughter could be added. Depending on your final number of adults and children, you would want to book two or three rooms.
If you would like to keep your whole family together in one accommodation, you can explore a Disney Vacation Club resort, such as
Disney's Old Key West Resort. It is very relaxing and the villas are spacious. A two-bedroom villa here sleeps nine adults plus one child under three years old, which would work if your party is 10 people and the tenth is your granddaughter. If you end up with the larger party, you could book at three-bedroom grand villa, which sleeps up to 12 adults. In each case, you would have a full kitchen, washer and dryer, and separate living space right in your villa!
Since you are looking to travel soon, I suggest that you contact Disney as soon as you can to make your reservation. You can call them at 407-W-DISNEY (407-934-7639).
Please do let us know if you have any further questions, Ann!