When we ate breakfast there last October, we did notice a few older girls (say, 7 and older) wearing princess outfits. But most I would say were in the 6 and under age group. Our daugher, wore shorts and a Cinderella t-shirt, which is what she felt most comfotable in and that seemed to be the more common attire. Now having said that, if she feels comfortable wearing the costume, then by all means go right ahead. She certainly won't look out of place. You're only young once, and after all you're in Disney World of all places eating in Cinderella's Castle !!!