docs: def _unset_child(self, name, child):
          """
          Untie child from parent.
  
          :param name: Child name.
          :param child: Parentable object.
          """
          if name not in self._children or self._children[name] is not child:
              msg = 'Child {child} with name "{name}" is not found'
              raise ValueError(msg.format(child=child, name=name))
          child._set_parent(None)
          self._remove_child(name, child)