 Why had the children never seen Earth?
"We face the unpleasant realization that what was feared is probably true—that worlds such as the Sun's are a rare accident, and that we may complete our search without finding even one new Earth. Certainly the building of this ship and its two sisters, the immense expenditure of time and labor and energy stores that went into them, left Earth's economy drained and exhausted. It would be a one-way journey—even if enough fuel remained, there would be little point in returning to Earth after more than forty thousand years. The stars had claimed his heart—but he was still a part of Earth, and not even nine hundred years of space and time had been able to alter that.
 They were born while on Quest III.
 The ship didn’t have enough fuel to return to Earth.  Earth was destroyed before they were born.  They weren’t able to see through the portholes.  They were born while on Quest III.