We experienced it last year October 30th and it was a BLAST! The parade is truly one-of-a-kind and it is (in my opinion) Disney's best firework display!
... and then there's the trick or treatin' for the candy *drool!*
Things to consider:
go back to your hotel in the middle of the day and take a good nap to restore all of your "batteries" (you parents will need the extra rest as well!)
before you leave (enter Can. city here) _____________ for DisneyWorld, go to your nearest Dollar store and pick up some neon glow sticks, flashing LED pins, or the like. It will save you some SERIOUS cash from the rolling kiosks that make their way up and down Main Street. Not only will your kids like it, but they will stand out from others by having "unique" flashing accessories while they trick or treat around the park.
mom, dad, grandpa, aunt Gertie - dress up as well! Part of the fun for me was watching whole families trick or treat together all dressed up. Some were even themed (very neat!) Just remember that deciding to buy costumes at Disney can be very VERY costly.
I could go on and on, but really, everyone on the boards have all said the same thing. This is one event Disney really does extremely well. Your family will have fun, you'll have fun and I believe that if you're doing this on your last night at DisneyWorld, then you'll going out with a BANG! A great way to end your trip!
Have fun!