There are two viewpoints:
It may refer to children that have also been exiled with adults. In this case it is a painful reminder that innocent children suffered the consequences of their parents’ disobedience (Exodus 20:5).1
It may refer to the population that went into exile. They are the children of Zion, and their exile left their mother alone.2
In either case, Jerusalem is left behind as a destitute widow and mother bereft of her children.3
5 Her foes have become the head; her enemies prosper, because the LORD has afflicted her for the multitude of her transgressions; her children have gone away, captives before the foe.