code: def _child_from_reference(self, reference):
          
          for child in self._child_to_parents:
              if self._get_sensor_reference(child) == reference:
                  return child
docs: Returns the child sensor from its reference.
  
          Parameters
          ----------
          reference : str
              Reference to sensor (typically its name).
  
          Returns
          -------
          child : :class:`katcp.Sensor` object
              A child sensor linked to one or more aggregate sensors.