WE DID NOT KNOW THEY WERE ALLOWED. So how could we inform them?
Generally, airlines are required to make accommodations for passengers with disabilities - which would include pet allergies. I would suggest calling the airline and asking that note be made in your record of the issue. If the agent doesn't seem to understand, the buzzwords are "Part 382" (the section of the Department of Transportation's rules that applies) and "Complaint Resolution Official" (airlines are legally required to refer you to a specially-trained staff member if there is a dispute over a disabilities issue).