Badgers? We no need no stinking BADGERS!
Well, if you are strictly talking murdering them... fine... but that isn't a challenge.
If you are looking for super soldiers, you will need some survivors, and the more the merrier.... you do have to put them through hell, but it doesn't have to be a random hell... it is better for it to be a meticulously controlled hell.
Too much randomness leads to dead soldiers, and wasted time and trouble.
Same goes for tantrums. They kill otherwise good raw material, and all the time and trouble you spent on them is wasted effort.
Hell, I say keep 'em happy.
If they are going to be forced to watch 1,000 people butchered, burned and impaled... why not let them do it from a front row seat in a legendary dining room with a masterwork picture window? Gonna bathe them in magma? Give them a nice reward by dousing them with a pleasant waterfall, and letting them recover on their artifact bed.
You will have far fewer tantrums, and will wind up with far more functional, efficient, ruthless, super soldiers.
That was how the Germans did it, and they STARTED the entire 'super soldier' fad anyway!
While we are looking at tried and true methods... let's look at another: training the children.
Obviously, anything with armor and weapons has to wait until they are adult enough for 'military training'... after all, we wouldn't want the lad to hurt himself, would we? That's our job.
Good news, here? Less wasted equipment. Bad news? You have to wait until they are adult to train them in their weapon and armor skills. Honestly, though.. if we are looking for a solid, consistent method of cranking out ruthless killers, then we shouldn't try throwing everything at them at once... rather, we get them ready and make sure they are killer material BEFORE we strap them into armor and give them weapons.
Danger room would have a low survival rate, anyway, simply due to the fact that they aren't wearing any armor... it would also not train them in weapons, armor use, or shield... just dodge.
This would probably have to wait until adulthood... but the rest of the training could be done to them pretty early.
I have heard, here, of using animals for training... I think that t be a mistake. The entire 'throw him to the wolves' thing seems to wildly unpredictable to me, and I imagine it ending up with horribly variable results. Most of them wouldn't even be useful, IF they survived. Also, again, this is really combat training... giving them any ability to fight back should wait until after you have their complete attention.
Instead:
1) Let them watch. It is never too young to start desensitizing them. Let them watch cute puppies and kittens being burned alive, spiked, sliced apart, whatever.. Put the butcher shop inside their barracks, then feed the animals to them; especially if it was a 'like'...
You like cats, little Tommy? Here, eat up!
Its too bad that you can't give them a profession yet, or you could make them take turns being the butcher.
In addition, they would watch each other die (probably pretty often, given the 'diet' plan, below). As another benefit of starting this early, you don't run the danger of them having to watch a 'lover' die, and have to deal with armed men going into a tantrum. Rather, they have had very little time to make 'friends' and the friends they do make happen to die pretty quickly. Even if they DO tantrum... its an UNARMED KID. Oh well.
I have a feeling (and, no, I am not a 'raw editor', so this is a hunch, not a fact) that these relationships build over time... acquaintance becomes friend becomes good friend becomes lover becomes spouse. It would not really be helpful for them to watch their friends butchered... simply watching an acquaintance die should be enough for the effects to be felt, and it helps eliminate tantrums. Besides, it might teach them pretty quickly that making good friends really doesn't pay off well.
The earlier you get them into the program, the greater their likelihood of graduating into real super soldier material.
2) Swimming.... nobody has mentioned it, but I would try and train my guys in swimming... just because you never know when some schmuck hits the wrong lever (or the right one, case may be), or whatnot.... and you generally want the strongest and baddest to survive.
Should be pretty easy to train up as well. Keep them in a room with flowing water at about 4/7.... enough to catch a breath, but enough that you will randomly hit spots of 5/7 or 6/7, and watch them turn a bit blueish. Should be no time at all before you can completely flood a room to kill a bunch of goblins, drain it and have the kid soldier still running around.
Besides, if you ever DO succeed at making a fire proof kid (not sure that extreme would actually be doable without RAW's), a good swimming skill would allow him to survive quite a bit longer when completely submerged in magma. Forget tossing cats down the tube... toss Tommy....
Speaking of which:
3) Fat. If they are going to die while on your 'Burn off those calories' plan, you might as well know now BEFORE you spend years on him. I vote for the wildfire idea, and teach them why not to play with matches. The wildfire seems more survivable to me (I could be wrong, experimentation will tell), and should still be effective... furthermore, you could do so en masse... just set the playground on fire.
Out of the training programs, here, this one is by far the one with the most random, uncontrollable results, and the one most likely to kill them off. Why waste time? Roll the dice first, and don't waste time on losers. Have them go through it as a group... then you train #3 and #1 simultaneously... and if you are working on #2, give them a convenient pool to dive into to put themselves out. Better chance of survival, and trains everything at once.
In fact, I would START with this... if you kill off 80% of your 'recruits', oh well.. they were going to die at this point in training anyway, and you may as well not waste the food, booze, time, or attention on some loser 5 year old that couldn't handle a little 3rd degree burn.
4) another one that I haven't heard mentioned... PAIN.
Granted, the fire would be painful... no arguments, there. This being said, it is probably far more controllable to drop them a variable number of z-levels to systematically train up their tolerance. By the time you have a 5 year old survive a 4z drop, you've got a winner.
The magma is good for pain, with an obvious problem... if they die 80% of the time, how many times can you actually dunk them? Good training requires repetition.
If you have a good doctor standing by, with good meds (and soap), they are far more likely to survive and benefit from the training... after all, who cares if they don't feel pain after they died? It isn't very useful, then. Let them break their legs, then heal from it... bust a few ribs, heal back up.
5) Fear.
Build a museum for them... trap creatures, and put them in glass cages... surround the kids with them.
At first, this will have to be done sparingly, or the kids won't go anywhere near them... this includes sleeping in bed or eating, if they have to go past them. Maybe put a single goblin in the corner of the dining room...
Assuming fear resistance can be built up, however, as they get better and better at running around these things, add more and bigger monsters.... maybe move the goblin into their bedroom.
By the time they are adults, they will be well-adjusted psychopaths; and ready to wield that ax with pride.