I read the reviews from the adult size, and from what they say, it runs small. They suggested ordering a size up. The petty-coat is optional. If I were your mother, I would suggest no petty-coat, and still would suggest shorts and leggings.
Deal is so right, order it now and if you do not like it you will still have time to return it and get a different one. I like her idea of going to a store and trying it on. LOL that is a great suggestion!!!!
These are just my suggestions, You don't have to take them. How about a nice costume that covers a little more. ;)