I have been running an experimental fortress lately, an attempt at a simple fortress that runs itself with minimal intervention on my part. This fortress has a large patch of plump helmets, a food storage room, a communal bedroom, and a communal dining room with a dozen wells and river pouring down onto a single-tile meeting room in the center. That's it. The farmers keep the plump helmet plot running, and everyone harvests and keeps the food stockpile full of mushrooms. They all eat cold mushroom and drink water, and spend all their time socializing in a pile of muddy dwarves under a waterfall.
A hands-off fortress like this depends on planters arriving as immigrants to replace the starting farmers when they die of old age. Over time the dwarves with nothing else to do with their time produce a lot of children, which fill up the population and prevent further immigration. To deal with this problem and to see if it was possible I built a device which removes children from the fortress. Dwarves walking to or from the food stockpile have to pass over a pressure plate which triggers for any friendly creature that weighs less than an adult dwarf. The pressure plate opens floor hatches that block the child from moving forward or backwards, while a side door opens to a path that seems to lead to the food stockpile. Down that path is a pressure-plate triggered retracting drawbridge that dumps the child into a corridor that only leads outdoors.
The device worked remarkably well. Very soon after it was installed a dozen or so children were wandering around outside. They could get back inside through the immigrant airlock, but would just end up being dumped back outside when they went to get food. Then an orc siege arrived, with predictable results. Now the fort has no children. Every time a baby grows to be a child - which happens pretty often as that many dwarves with nothing else to do make a lot of babies - it soon goes to get something to eat and ends up dumped outside.
I actually feel kind of bad about this. It's the worst thing I've ever deliberately done in fortress mode. Oddly the dwarves themselves don't seem to mind much. Apparantly the compounded happiness from standing under a waterfall with all your friends non-stop for years offsets having all your children torn apart by orcs and left to rot out in the open, while eating cold plump helmet and drinking water. I only had one tantrum, from a fisherdwarf who was new to the fortress and hadn't stood in the happymaking mist long enough.