foo.py

    @view_config(request_method='GET', name='children',
                 accept='application/json')
    def children(self):
        schema = FolderSchema(only=('id', 'title'))
        depth = None

        class Encoder(json.JSONEncoder):
            def default(self, obj):
                if isinstance(obj, Folder):
                    return schema.dump(obj)
                return super().default(obj)

        result = get_children_containers(
            self.request.dbsession,
            self.context.entity.id,
            max_depth = depth
        ).as_tree()

        self.request.response.content_type = 'application/json'
        self.request.response.headers['Access-Control-Allow-Origin'] = '*'
        self.request.response.text = json.dumps(result, cls=Encoder)

        return self.request.response