Hi Mariana! Celebrating a Disney birthday with your granddaughter will be amazing and so memorable for everyone.
I have two suggestions for character dining experiences for this celebration. If you wish to have something inside the Magic Kingdom then I suggest you book the Crystal Palace. This location has wonderful buffets for breakfast, lunch, and dinner. They can easily seat 12 guests and they have the main Winnie the Pooh characters stopping by each table. A two year old would love to see and interact with Pooh, Tigger, Piglet, and Eeyore.
If you are willing to take a short walk outside the park then you can consider booking something at Chef Mickey which is located in the Contemporary Resort. They also serve breakfast, lunch, and dinner and the characters are the "fabulous five". This is the best place to see Mickey, Minnie, Goofy, Pluto, and Donald all in one place. This is also a scrumptious buffet for all meals.
You can check out all character dining at this
Character Dining Site. You can check the prices and also menus for each location.