I'm trying to think of more ways to be even crueller than the stuff I've already thought of... but it's pretty hard.
Here's what else I've got:
Make another chamber near the first one, with all glass windows so that everyone, Dwarf and Elf alike can watch... make it like the
"Goblin Repeater" setup, with another door controlled by lever remotely that lets someone into that chamber, then close it behind them. Preferably one of the children spawned inside the fortress. The "bait" will just be an exit off of this God-forsaken Hell-hole you have stuck them in, which should be plenty enough. Now then, instead of just having a repeater that keeps changing something back and forth, put a couple wooden scourge traps in there.
Hopefully, this will allow a child to smell a whiff of that sweet, blissful thing called "freedom" for the first time in their life, and they will make a mad dash for the door. They will brave any scourging for a chance at escape, and run straight through that thrashing time and again, only to see the door slam shut right in their faces every time, and the pointing and laughing of dwarves on their drink breaks taking bets on how many "laps" they can complete, and their parents watch, crying as the runner slowly gets flogged to death, knowing nothing of peace or dignity for their entire miserable, yet blissfully short lives.
It's crueller to give them hope when we all know that there is none.
... Hmm... I think I might need help from this point on. As much as it scares me to say it, I think at this point, I'm about evilled out. Can someone post some good automated torture device ideas?