I went to Moguli to get a letter from Topapa. The third letter mentioned that the children had gone on an adventure. The fourth letter mentioned that the children didn't come back. After receiving the fourth letter, I went to Ur and talked with Topapa. I learned that the children had sealed the cave, so I sealed the second floor of the cave. Walking triggered the plot. It is not difficult to see three children surrounded by three fireballs (LV 18 or above).
Oh, and don't turn onions at this time. Attack, magic and defense are all weak. They almost became stronger when they reached LV50, and became the best profession when they reached LV90.
If there are other answers better than mine, it doesn't matter if you don't adopt them. Finally, I wish LZ happy!