Hi there,
What lucky girls to have an Aunt and Uncle that would take them to Disney World. I'm sure your going to have a great trip.
The most sophisticated character meals are the Garden Grill and Cinderella's Royal Table. Restaurants like the Crystal Palace and Chef Mickey's seem geared to younger guests, though I love them too.
The Garden Grill has characters like Chip 'n Dale, Mickey, and Pluto. I always have a great time with Chip 'n Dale. They are usually pretty funny. I think it helps that they have each other to play-off of. I think the girls will be surprised how much attention they get from the characters. Make sure you bring a camera and offer hugs. Also, don't mention to your nieces that the restaurant rotates. It will be fun to see how long it takes them to figure it out.
Cinderella's Royal Table is by far the most beautiful restaurant in all of WDW. It is also nice to be called "princess" by the waiters. Princesses come to your table during breakfast and lunch, but at dinner there is just a show by the Fairy Godmother. All meals include a souvenir photo package of your party with Cinderella.
Enjoy your trip. If any other questions come to mind...feel free to ask. -Michelle