The conditions in a defeated Judah will be so bad that the world will appear to have turned upside down. It is in that sense that the child would stand up to the ancient, a reversal of the cultural expectation that the older person has wisdom that the child lacks.
The people will look for someone to lead them, but there will be no one, for all will be in the same situation. The Assyrian invasion conquered all of Judah, and was on the verge of conquering Jerusalem when the Assyrian army left. Jerusalem did not escape Assyrian domination, only total destruction.
Thus, “Jerusalem is ruined, and Judah is fallen.” The cause for this disaster is that the people’s “doings have been against the Lord.” The sins of the people who have strayed from Jehovah have created the conditions that allowed this destruction.