If you have household rules that state you do not accommodate children, then the booking is invalid. Contact Airbnb and request them to cancel the reservation and it would be done with no penalty to you. Word of caution.... any guest who would agree to rules they've clearly either not read or don't respect, is not a guest you want in your home. If you are not already aware, any setting you have that limits guests to those over 12yrs is ignored by Airbnb, who will show your property to potential guests in search results, when they are searching for properties that accommodate children. Further... Airbnb have now removed this setting with no notification whatsoever to hosts... no consultation or written advice. As far as I am concerned this is unethical, dishonest and disrespectful. The only way you can get around this disregard for your rules and expectations is to turn of instant book and require potential guests to advise the purpose of their stay, number of guests and ages. No doubt Airbnb will find that discriminatory in some manner, but at the end of the day, it is your property, NOT Airbnb's.