Rule #1 - DO NOT HIDE THINGS FROM YOUR KIDS!!! When they find out, and they WILL find out, they will say "Dad, what's with the double standard???"
Be open about it. You don't have to smoke around them, I only smoke cigars outside the house and love to while sitting on the bench in front or on the deck out back. If they happen to come out, I'm not going to hide it from them. I do smoke my pipe in the house but only in my utility room. They don't come in there much, my son does to find out which one I'm trying out and which pipe I'm using but that's about it.
Hiding things from the kids that won't hurt them is never a good idea. My kids also know I wasn't exactly a "good" boy as a teen but they don't know any specifics other than I drank a little and smoked a little pot. They also know why I did it and why it was really stupid on my part. Honesty is the best policy because if you blow your credibility with your kids it's hard to get it back.