Hi Linda!
With regard to discounts, your best bet is to make sure that you have created an account on Disney's website
http://disneyworld.disney.go.com/ Periodically, Disney may send out special offers. Also although there is no guarantee that discounts will be available, start watching the website around October of 2013 for your dates. In my experience discounts, if available, will start to be released around October, November or December for that time frame.
The Disney Dining Plan is offered exclusively to Guests of the Walt Disney World Resort hotels . This is park of the Magic Your Way Vacation Packages. For 2013, there are 4 different categories in the Dining Plan. It's really a personal decision as to whether or not you think it's a good fit. The plans are convenient and affordable. I do think Disney's Dining Plan is a good idea for any Guest coming to the Walt Disney World Resort.
http://disneyworld.disney.go.com/planning-guides/in-depth-advice/disney-dining-plan/
Since your granddaughter will be celebrating her 10th birthday (double digits!), you'll definitely want to make sure that is notated when you make your reservations. Also, let the folks at check in know about her birthday as well. She will be given a birthday button and almost everyone will be wishing her a Happy Birthday along the way! Once you've arrived, let everyone know it's her birthday. You never know what surprises might happen along the way! For some special celebration ideas, take a look here...
http://disneyworld.disney.go.com/celebrations/
I wish you a MAGICAL vacation!
-Jacqui