I'm a huge fan of
MNSSHP. I go every year and have never once been there with kiddos! I've dressed up and not dressed up. If you choose to dress up, go COMFY! Be a doctor (scrubs), a farmer (overalls, bandana, cowboy hat), or, my personal favorite, "person-in-their-pajamas!!" Hee hee!
Agreed that you should trick or treat (yep--more candy at the end of the night!). And keep an eye out for the characters--they're dressed in their costumes, too! Also, some pretty obscure characters tend to come out at MNSSHP--I've seen all seven dwarves come out together!
The fireworks are fantastic, so be sure to watch them. And the parade is one of my favorites that Disney has ever produced.
The Haunted Mansion is super spooky on MNSSHP night, so do visit! Also, Disney offers (off and on) a ghost story steamboat ride on the Magic Kingdom steamboat. I've never been on this, but it's supposed to be fun.
Enjoy!! And if you can get a reservation at Liberty Tree Tavern for dinner that night, you'll see the characters in their colonial-age costumes: too cute!