The stories _____ (tell)by those children are very interesting

are told 意思是这些小朋友讲的故事很有趣

The stories __told___ (tell)by those children are very interesting
那些小朋友讲的故事很有趣。

如果句子补全了应该是
The stories _which were told____ (tell)by those children are very interesting。

told
因为是故事与说之间是被动关系
过去分词作定语 修饰the stories

The stories （told）by those children are very interesting

told 是被动语态
孩子们讲的故事很有趣。

have been told