This tip is coming out on Halloween so it seems appropriate to talk about Halloween.
I recently spent time in Montreal and the Quebec provincial government is embroiled in a controversy about allowing people to wear religious symbols at work. Halloween is not a religious event so those provincial rules do not apply, but some of the same concerns may apply to your workplace.
Allowing people to wear costumes with masks in public places creates some risk. The reason bank robbers wear masks is to avoid identification. If your workplace is open to the public, you may want to consider creating a policy around allowing full facial masks into your workplace.