Hi Amy! Happy Birthday to your daughter! It's great that she'll be celebrating her big day in Disney World.
The character meal that I would recommend to see Minnie Mouse would be
Chef Mickey's at the Contemporary resort. This restaurant serves breakfast and dinner - both are buffets. The atmosphere here is a lot of fun. At times the characters parade around the room and kids wave their napkins. You also will be able to see the monorail come through the resort every now and then! Chef Mickey's has a large selection of food and a huge dessert bar at dinner.
Another buffet breakfast that sometimes features Minnie Mouse is the
Cape May Cafe at the Beach Club resort. The atmosphere here is bright and casual and the characters are usually dressed in their beach clothes.
When making your reservations, mention your child's food allergies to the cast member on the phone. He or she will put a note on your reservation. When you check in at the restaurant, make sure to tell the hostess and/or your server about the allergies as well. Often the chef will come out and talk to you about what things your child can and can't eat.
As a fan of Minnie Mouse, be sure to take your daughter to Minnie's House in Toontown at the Magic Kingdom! Here she can she how Minnie lives and even meet her at the gazebo in the backyard!
I hope your daughter has a magical birthday at Disney World!