Airbnb currently offers the option to add guests as children (2-12years).
i.e. If the max number of guests allowed is 2, a booking could be made up of 1 adult and 1 child or made up of 2 adults.
In some of my listed rooms I have 2 single beds and 1 child bed. I say child bed because it would be too short for most adults but not for children.
There are options to add toddler beds and other arbitrary types of beds like hammocks and water beds, however there is no option available for a child bed under sleeping arrangements. If there was this option, I would be able to accomodate 3 guests, of which one may only be a child.
This would be extremely beneficial as I could then charge a discount for children because they do not use an adult bed and it would also make my listing more family friendly .