Walt Whitman interpretation? "And debouch to the steady and central from the offspring great or small."?

Whitman is probably using "offspring" in that final line to mean something like "jumping off point" or "starting point." Having perceived that nighttime moonlight is reflected sunlight, he is aware that death is similarly connected to or reflective of life. No matter where he starts, he is saying, no matter where he springs off from, even if his starting point is the contemplation of death, he will eventually arrive at (or emerge into) an awareness of what is "steady and central" -- an awareness of the pervasiveness and grandeur of life.