I have already written an article about what to wear to a day at a theme park see blog post but in just in case you dislike the items I listed, or you still had questions, here is what to NOT wear/bring.
NO Flip flips!
I admit I find these shoes comfortable for the beach or pool, but they are bad for your feet when walking around for a long time. Also you would have to take them off for all the thrill rides.
NO long pants in the summer!
Florida is very hot all day so I can assure you that you will be comfortable in shorts.
Although Universal and Disney have water rides, bathing suits tops or bottoms are not recommended and you might get some funny looks from other guests.
NO huge backpacks!
In my other post I recommended a backpack or purse, but only bring a reasonable sized one. You might have to fit it in the square lockers when you go on a thrill or water ride.
NO heavy items!
Leave the huge cameras at home if you are visiting a theme park for a family vacation.
NO new shoes!
Nobody wants blisters! Break in your shoes at home first.
Overall, just be comfortable. You went to a theme park to have fun so make sure that is your priority and you shouldn't have to worry about your clothes all day!