I believe that the host should have the ability to configure combinations of maximum number of adults/children per reservation.
For example: I have a listing with a King bed and a Twin sofa bed, where a couple with 1 child would fit comfortably in the king bed and another adult would fit comfortably in the sofa bed. So the listing can allow 3 adults plus 1 child, but only 3 adults if there are no children, because 4 adults wouldn't fit comfortably.
Currently, airbnb doesn't let you discriminate between adults and children when choosing max number of guests.