... I registered an account for the express purpose of posting in this thread. I've played Dwarf Fortress a few times, and am preparing to download it when I arrive at my proper computer, and have been skimming through the wiki and reading a few topics to try to orient myself a bit. I am still marginally terrified, and I am sure lots of fun will be had.
I fully endorse the mermaidapault in any and all applications (such as flinging them up to a large cup on a pillar to catch them in, which can then be flushed down into the fortress by a comically oversized toilet lever).
However, once the delicious merpeople are captured, I believe that the easiest way to harvest their infants is to chain them up inside a pressurized stream or river in your fortress. Once the babies are born, they will be carried downstream to cage traps, transported happily to the rotting fields, and then released to flop around a bit before they finally die. Perhaps. Thusly, the merbabies would be immediately delivered to wherever you could want them courtesy of the 'river', while the merstud and his mercubines happily fornicate in the rapids to ease the grief of watching their children's many skulls adorn the walls of the underground city they are captive in.