Look at pictures of ballerinas to get an idea of the key elements for the costume. Consider the classic white or pale pink tights with a black or pink leotard. Other elements include a tutu, which is extremely popular with little girls. Pink Ballet slippers are key, too, although, they might not be the best option for trick-or-treating. Hair should be pulled back and put up. There really aren't any extraneous props to be carried around. The costume and hair says it all. Props end up in the way as soon as the child leaves the house, so follow a "less is more" philosophy when planning the costume.