Heelys are not banned at WDW but when i was there we did try to tell the children not to skate on them. I saw so many accidents!!!
One of the worst was when i was on parade duty. There is a 30 second gap at a certain point in the parade where we can allow people to cross the parade route. Well one boy was on his heelys skating across the road when one of his wheels came off. He went on one direction and the wheel in the other. We had to get him from the parade route before the float and characters came too near.
Not only this, but there was now a heely wheel in the middle of the parade route which any performer could have tripped up on!!