docs: def get_children(self,node):
      """ Find all the children of a node.  must be a undirectional graph with no cycles
  
      :param node:
      :type node: Node
      :returns: list of nodes
      :rtype: Node[]
      """
      if self.find_cycle() or self.__directionless:
        sys.stderr.write("ERROR: do cannot find a branch when there are cycles in the graph\n")
        sys.exit()
      v = self.__get_children(node.id)
      return [self.__nodes[i] for i in v]