I propose a "Confidential Field" for each person in the tree which is viewable only by the site administrator. This is the reason: Suppose person A says, "Don't post my birthday on the site." Sometime later, person B says, "I know person A's birthday." Having forgotten what person A told me, I post his birthday and make him mad.
If there was a field attached to him which only I could see, I would put a note saying, "Mr. A doesn't want his birthday posted."