Set child.

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

def _set_child(self, name, child):
        """
        Set child.

        :param name: Child name.
        :param child: Parentable object.
        """
        if not isinstance(child, Parentable):
            raise ValueError('Parentable child object expected, not {child}'.format(child=child))
        child._set_parent(self)
        self._store_child(name, child)