Other forms: grandchildren
If you have a grandparent, you are a grandchild. You are the grandchild of your mother’s and father’s parents. If you have a grandchild, your son or daughter had a baby. Congratulations.
A person becomes a grandparent when their kid has their first baby, and that baby is the grandparent’s grandchild. Boys are often called "grandsons," and girls "granddaughters," but both are also grandchildren. Grandchild uses the family or genealogical sense of the prefix grand-, which first meant "a generation older than," and since Elizabethan times has also meant "a generation younger than."