The basic idea is this: In order to weaken your defenses, goblins send a wave of weak, expendable "troops" at you. These would distract and wound your military and clog up your traps. There are a few categories of cannon fodder they might use, described in order of complexity:
Imagine scores of crundles, badgers, or dark gnomes running at your fortress. Sure, the military takes them down in a couple of swings, or the marksdwarves get them before they reach the outer wall, or your traps turn them into little bits; but the military has some cuts and bruises, maybe a dwarf or two with a broken bone, your marksdwarves need to scurry off for ammo, and your traps are jammed with gnome bits. Suddenly the siege is much easier for the goblins. And all they lose are some pets and slaves bred just for this purpose!
Yeah, goblins have some laws. Treason is punishable by death, and...that's about it. Nonetheless, these convicts could join forces with goblins' POWs to form a poorly-armed (wooden staves, training weapons, maybe a bit of copper armor) force, which poses a more serious threat to the militia. They'd need to be dealt with. However, these forces might be willing to surrender after a few of their number are killed (or even before), as long as no non-traitor goblins are close enough to retaliate against them. The goblins you allow into your fort might betray you later, but as long as you treat traders from their mother nations well, POWs will probably be OK. If this doesn't happen, you could lose an important soldier or two, while the goblins only lose a dozen or two people who they were probably going to kill anyways.
This could be a punishment for unruly kids (goblin or abducted), or standard procedure for abductees, depending on the goblin civ. Child "soldiers" would be given leather armor and a knife, then sent on their not-so-merry way towards the fortress. Some kids would run away from armed dwarves; the dwarves would repay this by only attacking children who attacked them--other kids would be treated something like merchants, and only be attacked if you told them to. Dwarves should get bad thoughts from having to kill (non-goblin?) children, especially dwarven children, unless they were exceptionally callous. Children who don't attack the dwarves, whether you catch them in cage traps or if they just hide somewhere from dwarf and goblin alike, might be integratable into your fort, unless they run off into the wilderness. These child-soldiers would be top priorities for goblin snatchers.
So...what do you think? Too much, even for goblins? Other ideas of goblin cannon fodder? Etc? I'm never very good at this part...
EDIT: Did I hit post already? Modify message! MODIFY MESSAGE!