The best guideline we can offer is: do what's right for you, and what's right for your relationships. The ultimate purpose of self-disclosure, after all, is to enhance your effectiveness as a parent and a person.
Therein lies the rule of thumb: "Will my thoughts and feelings here, now, and with this child or other person, enhance me and enhance them and contribute to the quality of our relationship?" When the answer is "Yes," self -disclose. When the answer is "No," choose not to self-disclose; and when the answer is "I'm not sure," we strongly suggest that you take a chance and trust that your honesty and humanness will pay off. We're convinced that children especially thrive when their parent is a genuine, open and even vulnerable human being -- not an actor in a carefully controlled and rehearsed "parent" role.*
*Excerpt from Dr. Thomas Gordon's P.E.T. Participant Workbook