docs: def add_child_objective(self, objective_id, child_id):
          """Adds a child to an objective.
  
          arg:    objective_id (osid.id.Id): the ``Id`` of an objective
          arg:    child_id (osid.id.Id): the ``Id`` of the new child
          raise:  AlreadyExists - ``objective_id`` is already a parent of
                  ``child_id``
          raise:  NotFound - ``objective_id`` or ``child_id`` not found
          raise:  NullArgument - ``objective_id`` or ``child_id`` is
                  ``null``
          raise:  OperationFailed - unable to complete request
          raise:  PermissionDenied - authorization failure
          *compliance: mandatory -- This method must be implemented.*
  
          """
          # Implemented from template for
          # osid.ontology.SubjectHierarchyDesignSession.add_child_subject_template
          return self._hierarchy_session.add_child(id_=objective_id, child_id=child_id)