the.com/grandchild

means The child of your child — the next generation down, whom you get to enjoy without the full-time exhaustion of raising them.

from A transparent compound of "grand" + "child." The "grand" here doesn't mean splendid — it borrows the sense from French "grand" (great, big) used to mark a generational leap, just as in "grandfather" and "grandmother," which English assembled in the medieval period on a French model. So a grandchild is literally a "great-removed" child, one step further along the family line. "Child" itself is solidly Old English "cild," a Germanic word for a young person, with the slightly irregular plural "children" preserving an old pluralizing pattern.