If you are having a meal in the park, then park attire is just fine. You need to be wearing shoes and shirts.
Some kids dress up for princess meals in Sleeping Beauty dresses, etc.
For other restaurants, like signature ones in the resorts, the dress code is business casual. Men usually wear slacks and a collared shirt. Women wear casual dresses, capris, that kind of thing. But really, you see all kinds of clothing ...