I have no idea why the kid was in the cage in the first place - I haven't been using webbed cages until later (when I imprisoned a couple of web shooting FBs). And since children do not work they usually have enough time to find beds. Most probably the kid was injured by goblins and felt into a cage while retreating. A new memorial slab confirms it died of dehydration.
Putting the cage in a frequented area was not enough to "find" the body. Even dumping it was not enough, somehow the guy who was carrying it didn't notice. I have forbidden/unforbidden the body to be dumped onto the ground and it still wasn't enough. Finally I applied the usual technique of building a floodgate near the body, and the builder found it. The parents weren't amused, especially the father who lost two cats to a recent goblin ambush. Immediately after the revelation a band of zombies besieged my fortress, and killed another pet cat of the father. He's really unhappy now. I'd have tried other solutions, but I was just curious how the parents would react.