Creates a new child TransformRef with transform_path specified.
        :param transform_path:
        :param child_id:
        :return: TransformRef child

def create_child_ref(self, transform_path, child_id=None):
        """
        Creates a new child TransformRef with transform_path specified.
        :param transform_path:
        :param child_id:
        :return: TransformRef child
        """
        transform_ref = TransformRef(transform_path, child_id)
        self.add_child_ref(transform_ref)
        return transform_ref