great-grandchild       
noun [ C ] uk/ˌɡreɪtˈɡræn.tʃaɪld/ us/ˌɡreɪtˈɡræn.tʃaɪld/  great-grandchildren
 曾孙;曾孙女;曾外孙;曾外孙女
the child of your grandchild (= the child of your son or daughter)
  We have two grandchildren and one great-grandchild. 我们有两个孙子和一个曾孙。 
 He was the great-grandchild of Van Gogh. 他是梵高的曾孙。 
 - More examples
 - When she has trouble sleeping, she runs through the names of her 36 grandchildren and great-grandchildren.
 - There were two surviving great-grandchildren of Queen Victoria.
 - Her grandchildren and great-grandchildren still live in Rayon Park.