Shorts are fine.
while some people like to get more dressed up there it isn't necessary.
I think you are not supposed to have any type of bathing suit attire, tank tops or rag-like tees.
DH usually wears a pair of Khaki shorts and a golf shirt, especially since we have been in a park most of the day.
We have gone there prior to DS and once without thanks to my parents and did dress up a little more, but we are comfortable either way.