According to this KB article https://kb.blackbaud.com/knowledgebase/articles/Article/204376, the visibility of children to their parents in the Relationship area of the contact card is controlled by whether the student is visible in the Directory. These two things should be independent of each other. There is no reason that a directory setting, which is for public and internal school use, should override the visibility of a familial relationship. If a parent should not be able to view their student's information, the No Access option in Relationships should be used instead.