Maybe this year try to find a costume that is "book-related." Plan together the costume that your child will wear and then collect some books on the theme to read together at night. There are millions of books out there that can fortify the character and adventures of the "thing" or " person" that your child would like to be this Hallowe'en.
It will be a win-win situation when you put words to the costume.
You also can take photos of the before and after affects of the costume and write stories about the "great transformation".
Relax. Have fun and try not to buy into the scariest ones. When I taught Kindergarten and Grade One I tried to focus on the fun-side of Hallowe'en, not the scary part. Kids in other grades whose teacher's chose scary would get notes from parent stating that their kids were unable to sleep and that they were having bad dreams at night. Remember, young children are easily influenced by scary books and conversations as their imaginations make fictional stories seen real in their little minds. Hallowe'en is a great party time of the year. Please focus on the celebration and read on.