Although their relation likely will be discovered during roll call (assuming they share the last name), try to avoid bringing it up until you see how the parent and child react to each other. You're most likely dealing with a college-age student and her middle-aged parent returning to school, so anticipate that there may be some tension on the part of the younger student. Wait to see how the two interact in class before you interject any discussion of their relation.
There's no reason to approach parents and children in any other manner than you would typical students in the classroom. They are still regular students; they just happen to be related to one another. Maintain a professional but cordial atmosphere within the confines of the learning environment. If they don't sit together and never speak openly of their relation, you may never know the difference. However, feel free to ask them if they are related after class or in private.
If the two sit together and often joke with one another or speak openly about family matters, then you should feel safe bringing their relationship up in class and have fun with the situation. Not everyone's comfortable being in a class with his parent or offspring, but many probably find the situation humorous. Pair the students together for group work or studying. Having a relative in the class may help some students learn easier and quicker. In "The Joy of Teaching: A Practical Guide for New College Instructors," Peter Feline notes that, "having a close friend or relative in a class can often serve as a motivation and study partner for some students."
Avoid telling the parent or child how the other is doing in class. If the parent walks up and asks, in private, how her son or daughter is fairing in the class, simply reply that you feel it's inappropriate to share this information. Close relatives should share their problems with each other; it's not your job as an educator to share private information about a student regardless of whether the interested party is family. If the two have trouble sharing or interacting, invite them in for a joint conference, if they seem interested.