code: def add_child_objective_bank(self, objective_bank_id=None, parent_id=None, child_id=None):
          
          url_path = self._urls.children(alias=objective_bank_id, bank_id=parent_id)
          current_children_ids = self._get_request(url_path)['ids']
          current_children_ids.append(str(child_id))
          new_children_ids = {
              'ids': current_children_ids
          }
          return self._put_request(url_path, new_children_ids)
docs: Adds a child to an objective bank.
  
          arg:    objective_bank_id (osid.id.Id): the ``Id`` of an
                  objective bank
          arg:    child_id (osid.id.Id): the ``Id`` of the new child
          raise:  AlreadyExists - ``objective_bank_id`` is already a
                  parent of ``child_id``
          raise:  NotFound - ``objective_bank_id`` or ``child_id`` not
                  found
          raise:  NullArgument - ``objective_bank_id`` or ``child_id`` is
                  ``null``
          raise:  OperationFailed - unable to complete request
          raise:  PermissionDenied - authorization failure
          *compliance: mandatory -- This method must be implemented.*