When kids are preschoolers, all it takes to make a costume is a hat or a mask. In kindergarten or first grade, they need a whole costume, but they really believe in the transformational power of the clothing. If you have a real baseball uniform, you will be able to hit the ball. I once met a first grader who wanted a chicken costume so that he could hatch out a fertile egg.
By third grade, kids start to appreciate the humor of impersonating another character. The kids in grades 4, 5, and 6 make the most of Halloween and the chance to make fun of what scares them, whether it be monster, politicians, or the opposite sex.
A Halloween concert is a great idea. Nobody gets stage fright in costume -- if you make a mistake, nobody knows who you are, anyway. If you get nervous, it's probably because of the monster next to you. The kids sang Halloween songs, made good use of the percussion instruments, and practiced entering and exiting wearing all sorts of impediments. It was all great fun.