CiviCRM Managing Contacts in a Group
If I understand you correctly, parent groups work the other round to the way you are trying to use them: you don't add someone to the parent and then to a child group; but instead you add them to a child group and they are regarded as a member of the parent group.  

In your case, by adding someone to the Sales Executive-North group, they are automatically part of the Sales Executive parent group.  Similarly when you remove them from the child group, they are no longer part of the parent group (unless they were also in another child group).

As an alternative to parent groups you could use smart groups.