Keep in mind, it will be hot. I saw a lot of kids who were sweating to death, so I would suggest staying away from full-body costumes, or things that cover their heads, etc. You may end up carrying the costume around with you most of the night after your child begs you to take it off. I live in Wisconsin and our USUAL problem is finding a costume that's warm enough for Trick or Treating!!!!

So whatever I used for my kids' Halloween costumes at home was NOT going to work in this situation.
Anyway, last year was our first time to
MNSSHP and we weren't sure what to do. Keeping in mind the heat factor (and it was hot, stayed above 80 degrees the whole night, very humid), I originally just bought us all Halloween t-shirts at Target. A total of $20
Then the dress-up spirit caught me (a little) and I decided to do something a little more. Our family are big Wisconsin Badger fans, so we decided to dress as... Wisconsin Badger fans. DD5 wore her Badger cheerleader outfit (which we already had, and gets worn all year long), DS2 wore a Badger football jersey and carried a little football (which remarkably didn't get lost!) and DH and I wore Badger t-shirts. All clothes we already had or bought but would wear again all season long.
On the bus ride from the resort to MK, there were a bunch of guests from Michigan, so we engaged in a little school rivalry on the bus! Lots of fun...
As a side note, I saw LOTS of people there wearing the very same Target t-shirts I had purchased. So lots of people do it, but you won't be unique if you go that route. Still, it's very affordable, fits the spirit, and will keep you as cool as possible if it's hot!!!
Have fun finding something!