It seems that Staalo's system works quite well, but woudn't it be ideal if the children were subjected to mild injuries, such that they would train discipline, healing, etc?
I wonder if this could be accomplished relatively safely by using a coinstar pit with appropriate objects in it. If an object could be found which causes only bruises to skin, and nothing more, then the children would get constant injuries without any threat of serious ones. Some possible choices of objects:
1) training axes/swords - relatively light, very large contact area hits (it is correct that thrown weapons use their weapon attack properties rather than calculating contact area based on size?)
2) medium sized coin stacks (increase the number of coins until they just start to cause bruises)
3) clothing - not sure about this one
One issue would be that the smaller parts of the body (fingers, ears, etc) are more vulnerable, as the contact area of an attack is the smaller of the weapon contact area and the size of the body part (The momentum required to penetrate tissue scales inversely with contact area). Therefore, it might be beneficial to make the children wear mittens, boots, and hoods, but not pants or shirts. Then the clothing should protect the sensitive smaller areas (and train armor user) while the upper and lower body would get bruised by the flying objects.
Someone earlier in this thread had voiced the concern that wearing clothing would be a problem for coinstar, as then dropped worn out clothing would be a deadly projectile. Is this known to be a problem? If it is, will children respect burrows well enough that you could keep the new clothes separate from the training area, and then order the children into the clothing room periodically to change? This might also require an "airlock" room in between, as I believe dwarves tend to drop worn-out clothing whereever they are when they get the idea to change their clothes...
One other thought - is it possible to build pressure plates on the same square as the retracting bridges? I suspect not, but if it is, then you could imagine placing a pressure plate under each bridge, which triggers the opposite bridge. Then the children would trigger their own training, and more importantly training would automatically shut down if the test subject stops moving for any reason, allowing recuperation time if thrown objects prove a little too deadly.