code: def add_child_sensor(self, child_id, child_type, description=''):
          
          if child_id in self.children:
              _LOGGER.warning(
                  'child_id %s already exists in children of node %s, '
                  'cannot add child', child_id, self.sensor_id)
              return None
          self.children[child_id] = ChildSensor(
              child_id, child_type, description)
          return child_id
docs: Create and add a child sensor.