code: def children(self, parent):
          
          if parent not in self._parent_to_children:
              raise ValueError("Parent sensor %r not in tree." % parent)
          return self._parent_to_children[parent].copy()
docs: Return set of children of parent.
  
          Parameters
          ----------
          parent : :class:`katcp.Sensor` object
              Parent whose children to return.
  
          Returns
          -------
          children : set of :class:`katcp.Sensor` objects
              The child sensors of parent.