As the parent of two boys who have been 9 years old (and one who will be,) I'd say Garden Grill in EPCOT!! It's our arrival night tradition every time, and our boys' favorite WDW restaurant. It features Mickey, Chip/Dale and Pluto. We enjoy the food and the atmosphere is fun, but relatively relaxed for a character meal.
Now, Chef Mickey's in the Contemporary is classic Disney, so you might consider that, too. We've been there several times, and don't enjoy the food and atmosphere quite as much as Garden Grill, but I have to admit that Chef Mickey's is more classic Disney than Garden Grill.