My twins girls are 5 and we just returned from WDW. Currently, their favorite places for dinners are:
Chef Mickeys at CR ~ our best character meal experience with mickey and friends. Fun to see the monorail pass through as well.
Crystal Palace at MK ~ Pooh characters which my girls still love. Food isn't great, but my girls have an incredible amount of fun there.
CRT at MK ~ expensive, but my girls loved the experience of being inside the Castle with Cinderella for a meal.
Ohana dinner at Poly ~ served family style, we loved the food, time it right so you can see Wishes either from your table or from the beach afterward. They also do fun things for the kids during dinner.
Sci-Fi at Hollywood Studios ~ cool experience eating in a "car," but I wish they'd show other types of movies flicks, though I understand why they show what they show. It's science fiction-type flicks which sort of scare my girls, but the whole experience is very cool and my girls talk about it a lot.
Coral Reef in Epcot ~ very unique restaurant where the entire wall is an acquarium. Last time, we watched a large turtle and a shark swim around for our entire meal.
Liberty Tree in MK for lunch ~ if you want excellent food at a casual place, have lunch at Liberty Tree. Food is outstanding for lunch. Dinner is a character meal served family-style and we didn't enjoy the food as much. We make it a point to have lunch here every trip.
The Boardwalk area has a lot of fun and good places to eat if you happen to venture there. ESPN Club if you like sports, Spoodles is a good restaurant, and Beaches & Cream at the Beach Club is an old-fashioned 50's-type diner that serves really fun ice cream treats.
I notice a lot of people are recommending Whispering Canyon at WL. We just returned from there and while it's an awesome restaurant (and resort!), my 5 year old girls didn't like the antics at WC. It is probably just a combination of their age and personalities, but they said they didn't want to go back LOL! However, we will give it another try in a couple years.....