In the case where someone already has a gun out as part of a costume, there would be less time to act, than if they had one inside a backpack.
So your FIRST reaction would be to call law enforcement?
If so, then you are a victim of the insane fear that has gripped so many in our nation. Your FIRST response should be to resolve the situation yourself, especially if you are in a leadership position.
If you can’t do that, or don’t know how, or are paralyzed with fear about what might be, then I sure don’t see how you could be responsible for the safety and well-being of hundreds of children.