Definitely look at the weather forecast right before you go. That will help you get a general idea of what it's going to be like this year.
You'll probably want to take a combination of pants and shorts. 1 or 2 light long-sleeved shirts for each person would also be good. Hint: wear a short sleaved shirt under your long sleaved shirt so that if you have to wear it again it's not gross. (The kids may need a few extras if the forecast looks cool since they're more likely to spill.)
Take multiple pairs of shoes in case they get wet in rain. If you take sandles, make sure you've gotten them wet and walked around for a while before to ensure you don't get blisters and that the color doesn't run on your feet.