We did it 3 yrs ago. We were dressy casual. I do not know that there is a specific code or not though. I know I wore a very casual sundress and sandals. DH and DS had khaki type shorts and polo's. Thats what they usually wear anyways.
I have been there many times and it is very casual. If you are coming straight from the parks, make sure you are not all sweaty though, the seating is really close together.