Hi Sarah! It sounds like you have a great vacation planned which will be even more fun with grandparents joining you!
I've listed a few character dining meals that I would recommend for your family. Please note: I've included the characters normally seen at these locations, they are subject to change.
Crystal Palace (Magic Kingdom- Pooh, Tigger, Eeyore & Piglet)
Chef Mickey's (Contemporary- Mickey, Minnie, Goofy, Donald, Pluto)
Donald's Safari Breakfast (Animal Kingdom- Donald, Mickey, Goofy & friends)
Cinderella's Royal Table (Magic Kingdom- Cinderella and Princess friends)
1900 Park Fare Breakfast (Grand Floridian- Alice in Wonderland, Mad Hatter, Mary Poppins)
Breakfast at Akershus in Norway (Epcot- Various Disney Princesses)
Breakfast at Cape May Cafe (Beach Club Resort- Minnie, Donald and Goofy)
Breakfast or lunch at Playhouse Disney's Play and Dine (Hollywood Studios)
I'd also suggest 'Ohana in the Polynesian. This dinner is very entertaining for everyone. Your oldest daughter would likely enjoy the coconut rolling and hula dancing lessons :)
Have you considered the Hoop-Dee-Doo Musical Revue? See
HERE. This show is a LOT of fun for adults and kids alike. Your little ones might be too young to "get" a lot of the jokes, but the atmosphere is still a lot of fun.
Be sure to pick up a complimentary Birthday Button for your daughter at any Guest Relations counter in the Parks.
Hope this helps, and have a magical vacation!
Kaylene