FWIW, my DD never wears actual undies at theme parks in hot weather. She wears the sports shorts or skirts with the built-in panty, all wicking, and much less to pack.
Also FWIW, cotton tighty-whities for the guys are banned when I'm doing the packing. They are thick and don't dry when they get wet in the parks, and they take up a ridiculous amount of space in luggage. They tend to wear swim shorts with the mesh liners in the parks, but any actual underwear that comes along on travel is the wicking kind.
Cotton underwear is great for normal wear, but when you are constantly walking around in wet underclothing, it causes chafing, and the omnipresent moisture causes bacteria to multiply. Wicking is best for theme parks.