@Erin6941 Forget Airbnb, in time you will appreciate why I say that. It is best for you as the owner to address any issue yourself and on the spot.
You sound kind-hearted (and gentle enough) which suggests you will not in all likelihood come across too 'heavy' by mentioning the fact they brought unexpectedly 2 extra dogs and ask ''Hey folks I see you brought 2 extra dogs and is it cool by you to pay a bit extra for them, Lord knows takes a lot more cleaning with animals. It is just 'X' amount.". 😎
Two more steps are left after these folks leave, regardless of what their position is:
1. Make sure you spell out in your description somewhere/somehow that you do need to know how many people/pets are coming before guests arrive - so this doesn't happen again.
2. Best not to divulge how you know they brought extra dogs, but this brings up a reality: the best way in hosting is to meet the guest when they arrive, in this way you can bring up any pending issue before they walk inside the door. Always with in a light manner.