For Girls: Spaghetti strap in white or black (PLEASE DON'T WEAR GRAY) Or a light colored cotton crop top, because polyester would be too hot. If you can find similar items that are Disney themed PERFECT wear them. Stretchy shorts (something comfy) you will be walking a lot, so some type of comfortable sandal. I would recommend wearing your hair up or out of your face so it doesn't get too tangled on roller coasters, in preferably a braid or bun especially if you have thicker hair. Light makeup, because it will wear off in a millisecond. Just use the basics. Concealer, and if you want to do lashes its a waterproof mascara or fake lashes. If you cant live without a full face, that's fine.
For boys: Cotton Jacket (If you get cold. My brother is always cold), A baseball hat. Some type of Spandex/Polyester mix T-shirt (NOT GREY). Any Shorts. Comfy Sandals. OK now you can go to Disney!