A grandchild was a name used to describe one's descendant, namely the child of one's child, where the subject in question would be the grandchild's grandparent. Each subsequent generation of grandchild was given the qualifier of "great", which was then compounded. (ENT: "E²"; DS9: "Dax", "Children of Time")
Tom Paris suggested that although the astronomer had been dead for many years, they could send a message to his great-grandchildren instead. (VOY: "Blink of an Eye")
Familial relationships
Female
A female grandchild, or the daughter of a child, was called a granddaughter.
Beverly Crusher was the granddaughter of Felisa Howard. (TNG: "Sub Rosa")
Els Renora had a great-granddaughter who was about the same age as Jadzia Dax in 2369. (DS9: "Dax")
Male
A male grandchild, or the son of a child, was called a grandson.