We wore what we were wearing in the parks, but it was in the fall and we had on pants and decent shirts/shoes. I wouldn't wear shorts and flip flops or a hat. Some people were kind of dressed up. If you have time, change into something else in your room before arriving. Of course, this is just my opinion - others may disagree.
Have a great time!
You can wear what you would wear to the park. A lot of people do change for dinner. Nothing fancy, but pants and a shirt. The last time we were there, my sister and I did not have time to change out of shorts. There was no problem at the restaurant. I don't think that they care about what you wear as long as it looks decent.