The core of the problem is the fragility of the children test subjects. Anything dangerous enough to provide useful training is also dangerous enough to give a parent a "Lost a child to tragedy recently" thought in short order.
If I can capture one, I intend to experiment with werecreatures, as I believe their regeneration is the key to the 'fragility' problem.
Step 1: Isolate the test subjects as quickly as possible to prevent friendships and acquaintances from forming. This phase is fairly pleasant - each subject will be given as many luxuries and happy-thought generators as possible without compromising the test or annoying the nobles.
Step 2: Capture a werecreature. Preferably more than one. Ideally, this should be a fairly non-threatening werecreature - a weregopher or something. While were-badgers would be spectacular, we need something that a test subject is capable of surviving contact with. Unfortunately, I'll probably have to settle for whatever I can catch.
Step 3: Chain the werecreature in the center of a 4x4 room with one locked exit door and a retractable bridge roof. The door leads to a training cell, which will be filled with turkeys, wild dogs, whatever. This ensures that there is no 'safe' tile in the room.
Step 4: When the full moon hits, drop the test subject into the room. Keep the door locked until the subject is either bitten or a moot point. Assuming the subject survives, unlock the door and let the child flee into the perceived safety of its new cell.
Step 5: Proceed with training. Every month, the subject will turn into a werebeast, probably killing all the training animals, but more importantly regenerating all wounds. It might be useful to have a 'emergency escape' option for the cell to allow critically wounded test subjects to flee to a safe place where they can be exempted from testing until after the next full moon puts them back at full health.
While it might be possible to micromanage the system to keep the livestock alive, that's more of a pain than it's worth to me. The whole point is to keep the test subjects out of the way while simultaneously training them to be useful. I'll probably just breed as many turkeys as my fps can handle and pasture them directly over the training cells, dumping them in whenever a cell gets too empty to be useful.
One possible deal-breaker is that I'm not clear on what happens to children who are bitten. We know they can contract vampirism, but can children receive a werebeast curse? Another problem is surviving Step 4. I have no idea how often werecreatures 'bite' as opposed to other attacks, and I have my doubts about how well a child can survive even limited contact with one of the weaker werecreatures. There is a good chance that nothing will survive to reach step 5, which would require scrapping the whole plan.