I'm with the don't mention it side. First what you do on your own time is just that. Many workplaces have a no smoking on the job policy and those people are free to smoke on the way home from work or once home if they want to. This was after hours. It isn't like she caught you standing on the porch smoking while the kids were playing or even in the backyard standing at the side corner of the house smoking while the kids played. It was your own time.

The fact they are ok with your hubby smoking outside away from the children means they are ok with smoking in general and that helps. If anything is said then a quick explanation that occasionally you have a smoke with a drink while alone with hubby and then change the subject.