@Shaun69 This appears to be a weak link at Airbnb Support - Customer Service reps understanding insurance issues and limitations. I had a couple Instant Book and inform me afterwards that they were bringing an infant. I told them we needed to get the infant registered even though it was staying for 'free' (courtesy of Airbnb's rather generous policy on this). I was not able to register the infant - the Computer Bot kept denying the adjustment, saying 'Infants stay for free, you don't need to change the reservation', or something like that. However, I am aware that no one is covered under the Host Protection Guarantee unless they are registered. I imagine that this includes infants. I called Customer Service and an obviously new rep immediately said, "Don't worry, the infant is covered." I asked him to quote the policy or Terms of Service stating that an unregistered infant would be covered by Airbnb's Host Protection Guarantee - and he could not. I asked him to find someone who could, perhaps his manager. He could not. I was on hold for quite a while, and next thing I knew, the rep had called my guests to tell them they needed to cancel the reservation and do a new one and include the infant, all the while still telling me the infant was covered by the Host Protection Guarantee.
To this day I don't know if unregistered infants are covered or not, but given I also have my own commercial insurance, I am quite certain my property insurer would like ALL guests registered, including infants. So I state this in my House Rules. That ALL guests, including infants, must be registered at the time of booking.