So below is what I previously sent to the guest from Airbnb's Help, he is stating that he booked his inquiry correctly and because the way airbnb defines infants they are free he is not accepting this wording as being able to charge for an infant that it only counts for children over 2 years.
The real problem here is that guests use the system correctly by choosing infant and they believe since they did and it calculated their rate they believe any other change you are asking them to make are improper even if you have placed it in your house rules and prebooking messenge.
Since this is the second time this has happened I thought maybe there is some other wording that would end discussions like these instead of calling Airbnb.
Airbnb Help Start
Can children travel on Airbnb?
Yes, children can travel on Airbnb, but some hosts have specified that their space may not be safe or suitable for children or infants. Infants (children under 2 years old) aren't counted as guests when you're booking a reservation and don't incur any extra costs. Some hosts count children as guests, which can add an additional guest fee to the reservation.
Airbnb Help End