Sea monsters - Rather, jackals.
Their young ones - “Their” whelps. The term is applied only to the young of dogs, lions, and the like.
Related Commentaries of Lamentations 4:3
Lamentations 4:3
3 Even the sea monsters draw out the breast, they give suck to their young ones: the daughter of my people is become cruel, like the ostriches in the wilderness.