I think it would be highly appropriate to dress as a saint, as Halloween is the eve of All Saints’ Day. It would also be a nice counter-statement to all of the witch and demon costumes.
My favorite costume ever was my angel costume when I was 6.
I could be wrong, but I think I learned in elementary school that the custom of dressing up on Halloween came from children dressing up as their patron saint on the eve of All Saints Day. I think it would be very appropriate to dress as St. Michael.
DISCLAIMER: Catholic Answers has turned over the archive to Catholic-Questions.org and no longer owns, manages, or moderates the forums. For additional apologetics resources please visit www.catholic.com.