foo.py

    @view_config(request_method='GET', renderer='json',
                 name='hierarchy', accept='application/json')
    def hierarchy(self):
        schema = FolderSchema(only=('id', 'title'))

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

        result = get_children(
            self.request.dbsession,
            self.context.entity.id
        )

        self.request.response.content_type = 'application/json'
        self.request.response.text = json.dumps(result, cls=Encoder)

        return self.request.response