Renting to a couple with a pug dog

As a landlord, I've never rented out to families with pets. My listing says that as well. However I had a family apply to see the house and they mentioned that they have a 20lb pug that's 11 years old. My listing has been up for more than a month and hasn't been getting a lot of traction. I am now considering if it's really such a bad idea to rent it out to this family. They have no kids and it's just the husband, wife and the dog. 

I never had any pets and am not sure what to expect. Is it a good idea to start allowing pets? Are pugs well behaved (they said it's old, blind and just lays on the couch). How much pet fee and/or pet rent do you suggest I charge?