So I've been mulling over the science of this thread for a few months, and think I have the best design style yet.
First is a bit on understanding dwarven child psychology...
1: Dwarven children will do VERY few tasks. The only things they see fit to participate in, are deconstructing terrain and picking crops if this is enabled for everyone - these are the only two truly 'no profession' jobs. All other jobs, including pulling levers, are assigned.
2: Dwarven children do not respect burrows. When assigned, they will not go. When an alert is given, they will not go. If they somehow end up inside a burrow after being assigned/alerted, they will usually stay there until they become hungry, thirsty, or sleepy, at which point they'll leave. Adult dwarves will leave a burrow when starving to death, but children will leave when feeling a bit peckish.
3: The 3 things that dwarven children do respect, are bedroom assignments, meeting zones, and their mother. If left with nothing to do, they will go to their bedroom. If there is a meeting zone present, they will go to it. If there is nothing to occupy themselves, they will follow their mother or wander the fortress.
Second is some understandings of supply logistics...
1: A dwarven child will need approximately 100 food and 200 booze to survive for 12 years (overstocking is safer).
2: Dropping food/booze onto them will eventually be fatal.
3: It's possible to fall down a well.
4: A food stockpile is required to prevent food spoiling.
Third is concepts of trauma and coping...
1: Combat generates negative thoughts.
2: Sobriety causes negative thoughts.
3: Keeping mood elevated is of utmost importance.
4: Having an expensive bedroom, dining hall, or other room can improve mood, even if not being actively used - or if most of the room is even inaccessible.
5: Mist is an incredible mood elevator.
Fourth and final is combat...
1: Injury is rarely an issue, infection from small cuts is what turns out to be the most fatal.
2: Sufficiently large animals can still kill from outright injury.
3: Aggressive animals will cause children to panic, and will attack with extreme frequency and bloodlust.
4: Skill gain is not based on the type of attack incoming, but rather the fact that there is an attack at all.
What do we conclude with?
The interior chamber is a 1x3 room (3x5 including walls) with 1 bed, 1 food stockpile, and 1 meeting hall.
At least one wall needs to be a wall grate or fortification.
Plumbing is essential, as that open wall will allow for mist to seep in from a (probably) artificial waterfall.
Despite the obvious design option, you should NOT build two mirror chambers with fortifications facing the same waterfall, as the children will then be able to see and socialize with each other. Side-by-side housing is acceptable, but face-to-face is not.
If required, add an adjourning room next to the cage room, filled with lavish furniture. BEFORE finishing the construction, expand the bed into a room encompassing the expensive furniture, then add walls to separate the room in two - the room will remain in the same formation, so as long as you don't resize the room, it will continue to cover tiles that it can't access.
Dump in a great deal of food and booze.
Assign a food stockpile under it.
Assign a pasture, and put a few turkey or peacock.
Assign the bed as a bedroom, and give it to the child.
When the child walks in, have a mason arrive to seal him in - we're going full Cask of Amontillado here, doors would not be suiting.
Delete the pasture, and instead make a meeting zone.
From left to right, you should have a meeting zone, stockpile, and bed, with a set of fortifications probably along the length of the room to let in mist (or perhaps built L shaped).
The animals will naturally move to the meeting zone, as will the child, causing the animals to become cramped and attack randomly.
The food stockpile keeps the child alive.
The bed allows the child to sleep away from the animals, which is more important than you might think.
Using TAME animals allows the child to step away without panic, as sustained attack can prove extremely fatal.
The mist should help with infections and cleanliness.
It may be worth building a rail system overhead to deliver good through a hatch directly above the food stockpile (where the child is unlikely to stand) in case food/booze is miscounted or rots.
Keep in mind, this is extremely slow training for extremely long-lived fortresses. Skill and stat gain will be slow, but should be steady, and most importantly keep the child out of adult hair until it can prove useful.