code: def _child_as_list(self, pk, ck):
          
          return [self._values[pk][ck][k]
                  for k in sorted(self._values[pk][ck].keys(),
                                  key=lambda x: int(x))]
docs: Returns a list of values from the child FlatterDict instance
          with string based integer keys.
  
          :param str pk: The parent key
          :param str ck: The child key
          :rtype: list