code: def get_child_objectives(self, objective_id):
          
          # Implemented from template for
          # osid.ontology.SubjectHierarchySession.get_child_subjects_template
          if self._hierarchy_session.has_children(objective_id):
              child_ids = self._hierarchy_session.get_children(objective_id)
              collection = JSONClientValidated('learning',
                                               collection='Objective',
                                               runtime=self._runtime)
              result = collection.find(
                  dict({'_id': {'$in': [ObjectId(child_id.get_identifier()) for child_id in child_ids]}},
                       **self._view_filter()))
              return objects.ObjectiveList(
                  result,
                  runtime=self._runtime,
                  proxy=self._proxy)
          raise errors.IllegalState('no children')
docs: Gets the children of the given objective.
  
          arg:    objective_id (osid.id.Id): the ``Id`` to query
          return: (osid.learning.ObjectiveList) - the children of the
                  objective
          raise:  NotFound - ``objective_id`` is not found
          raise:  NullArgument - ``objective_id`` is ``null``
          raise:  OperationFailed - unable to complete request
          raise:  PermissionDenied - authorization failure
          *compliance: mandatory -- This method must be implemented.*