Hello Amy from NC from another Amy from NC,
Mickey's Not-So-Scary Halloween Party is one of my very favorite events at Walt Disney World! On the web site they do ask the following:
"Please note: For the safest and best possible Guest experience, please choose costumes that are not obstructive or offensive."
I think good advice is to not bring anything to the parks that could be mistaken in any way for an actual weapon. You will also want to make sure any masks or helmets are not too obstructive. My own personal advice? Remember that whatever you bring into the parks, you will be carrying around for the duration of the party. A larger, heavier item might seem like a great idea for a costume and photos, but the reality of carrying it around can put a damper on a fun evening.
Thanks for stopping by the Disney Parks Moms Panel. Be sure to let me know if I can help you with any other questions!
Wishing you a magical vacation,
Amy H