Do baby/child dragons breath fire?
Do they even have the stage baby or child for that matter?
The thought of a couple hundred baby dragons the size of Geckos breathing fire on Goblin/Kobold invaders makes me smile.
Dragons don't really have a "child" stage; that's reserved for sentient races. They do, however, grow larger as they age; the biggest dragons don't reach their max size until they're 1000 years old. I believe they breathe fire at any age.
However, making them into a reliable defensive option is difficult and dangerous. Containing them behind fortifications will cause it not to use it's breath on enemies, as creatures only use breath and web attacks if they can path (as demonstrated by the well-known GCS farm layout). Putting them on a chain in the open makes them vulnerable to archers, letting them roam around will result in crispy dwarves. I'd say your best bet is either the DodgeEm
TM trap with the dragon pastured below (more fun if creatures can target across z-levels) or using pasturing and creative maze building to lure enemies into the dragon's range.
The only way I succeeded in weaponizing a dragon was with a sacrificial system; my dragon was chained into a fortification cage at the end of a hallway, like so:
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O O O O O O O O O #####
<-out O O O O O O O O # D X to fort ->
O O O O O O O O O #####
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Above the pen was a hatch and a cage with all the fort's kittens and puppies. When enemies came, I'd lure the goblins down the path (the pillars were to keep archers from getting lucky shots through the fortifications), then once they were at the right point, drop a small animal into the pen through the hatch. The dragon, with prey in it's grasp, would breathe flame once or twice before tearing the pet to shreds, also resulting in many flaming goblins. Unfortunately, any that had shields typically blocked without a problem, and thus it's not perfect. Also, the first test-fire resulted in the death of the fort's only trained architect.