We just were there and saw everything being worn from cocktail dresses and full on suits to denim shorts, tank tops, and flip flops. I wore a summery casual skirt and my boyfriend and step brother wore khaki shorts and polos and we felt like we were perfectly dressed.
I've seen it twice. Both times I felt a bit overdressed and I wasn't wearing anything real fancy. Although it's Disney people feel as though they were the most casual of outfits (cut off shorts). If your spouse wears a pair of khakis and a polo shirt he would be fine. You could still wear your sundress. After the show, you are dressed and ready for Pleasure Island or House of Blues. La Nouba - there is a lot to see and hear. The show is taking place everywhere in the venue.
Have a great time.
I agree with the others. There was a huge mix of what people were wearing. Just wear whatever you feel comfortable in. There will be others doing the same.