Goblins are merciful, kind creatures, and you are torturing them for doing the children a favor.
yes, they are doing the children a favor, they are trying to save the fortress from a childsplosion, but it is not need. For i am also doing my fortress a favor, by smashing and slabbing children and by liberating them from life behind their parents' backs i free them from a life of labor and violence.
where the goblins and i differ is that the goblins tend to fail, and when they succeed they cause the parents to suffer because they werent discrete enough. However, my child-control attempts tend to succeed and there is no pain in a few spirited away children when no one saw them "leave".
the goblins' intentions do not matter, the result is still pain and suffering for dwarves due to the incompetence of their child snatchers, their attempted ambushes and sieges cause death none the less despite their intentions. And though i do try my best to get to dwarves with many friends before the goblins do im only a humble overseer and only capable of so much micromanagement. The occasional popular dwarf is killed by the goblins before im able to smash them, and this sometimes sends my fort to the edge of a tantrum spiral.
Important dwarves who are tantruming are sent to rehabilitation rooms, but i only have so many rehabilitation rooms...and am forced to reluctantly smash the rest out of sight to prevent tantrums from spreading any further. So as you can see, the goblins are not doing my fort any favors, only causing pain and suffering despite what their intentions may be. For this they will be punished with the utmost brutality, they must feel physical pain equal or higher in proportion to the amount of emotional pain my dwarves felt. (i mentioned higher because of the matter of deterrence)