@Liz76 If the child is under 2 then you can do nothing as they 'don't count' in Airbnb land. If the child is over 2 and your limit is two people then you can refuse them entry but you will have to fully refund the guest (however mad that sounds given they have broken your rules).
Many thanks for your reply. I managed to talk to someone at Airbnb and they have canceled the booking and issued the guest with a full refund. I'm still really cross that the guest was able to put me in this position she wasted my time and potentially lost me another booking.