It's easy for you to do it @Burke10. In their reservation details, scroll down to Support -> Change reservation. From there, you'll see this:

Choose "Change reservation," and you'll see this:

Click on the dropdown under "Guests," and you'll see this:

Choose the number of pets and submit the change for the guest's approval.
PS: In this example, the "plus" sign is grayed out because this listing doesn't accept pets.