Albert Barnes Commentary Lamentations 1:5

"Her adversaries are become the head, her enemies prosper; For Jehovah hath afflicted her for the multitude of her transgressions: Her young children are gone into captivity before the adversary." — Lamentations 1:5 (ASV)

Are the chief ... prosper - Or, “have become the head”... are at rest. Judea is so entirely crushed that her enemies did not need to take precautions against resistance on her part.

Children - That is, “young children,” who are driven before the enemy (literally the adversary), not as a flock of lambs that follow the shepherd, but for sale as slaves.