Untie child from parent.

        :param name: Child name.
        :param child: Parentable object.

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)