"Great-grandchildren" Natural Recordings by Native Speakers
The term "great-grandchildren" refers to the children of one's grandchildren. In other words, they are the youngest generation of one's descendants, being the offspring of one's grandchildren.
The word "great-grandfathers" refers to the grandfathers of one's grandparents. In other words, it is the generation of one's grandfathers' fathers. It is a term used to describe the paternal ancestors of one's parents, which is three generations prior to oneself.
The term "great-grandparents" refers to the grandparents of one's grandparents. In other words, they are the parents of one's grandparents, and thus, the great-aunt and great-uncles, aunts and uncles, are one's first cousin twice removed. The term is used to describe an older generation of ancestors, five generations away from oneself.