@Susan4461 Put in your house rules that all persons, including infants, are included in the guest count and must be disclosed and included as part of the reservation. This will help protect you in future.
The problem w/declining people at the door is that they can leave you a review. So, the goal would be to prevent such a situation.
In the future, you can send a follow up message to guests. 'Thanks for choosing our listing, blah blah, details of check in, whatever. We also wanted to confirm there will be X number of guests for your trip.
Of course some people won't read or respond or may lie, but most people, if they're bringing an infant, will mention it at this time. From there, you can explain the reasons why you have the property listed as unsuitable...whatever they are...stairs, no childproofing... You can invite the guest to cancel or withdraw the request.