Don't introduce them to this idea by lighting up. In some pleasant, relaxed situation, I would off-handedly mention that you are trying out a pipe to smoke now and then. You're an adult, so do not approach this like you are asking their permission. You are just sharing a minor fact of your life.
If they are negative, slightly, or completely horrified, give them much more time, and mention it again in months or a year from now. If they are only mildly disapproving, let it drop, but mention it again in some weeks. I'd give them plenty of time to adjust, and then eventually, if they are accepting, you could announce that you are stepping outside their place to smoke your pipe, and then come right back and continue with regular family activities.
Be respectful, be polite, be affectionate as possible, and see what happens. If they are totally rejecting on an ongoing basis, then just don't smoke around them or mention it much in the future. Maintain other things you enjoy having in common with them. You'll always need your parents in an emotional way, and they will always need you, so leave room for that. Your parents and your children are a life-long thing, as is family in general. Work with it as best as possible. There will always be points of irritation; hey, it's a family.