Hi there, Cheryl!
The costuming at Mickey's Not-So-Scary Halloween Party runs the gamut from highly elaborate creations to t-shirts reading "This IS my costume!" There are all types of folks dressed in all manner of fun outfits, and you ill find that Disney has a fairly liberal policy on adults dressing in Disney character costume.
The rule of thumb seems to be that if your costume is
too good, you might be asked to change clothing or amend your costume in some way.
You also should not behave like a character: posing for pictures or signing autographs. That can be a cause of confusion.
Most costumes are going to work out fine for the party, and certainly small children are not really at issue here. You'll just want to be mindful of any costumes that obscure vision, may cause tripping, impede hearing, etc. And for sure, nothing too violent or scary (no realistic-looking weapons are permitted).
You can find more of the guidelines on
the official page for the event.
Have a spook-tacular time!
See you later, celebrator,
-Deborah B.