Board Thread:General Discussion/@comment-25133718-20140703035309/@comment-3146930-20140703044022

The simple answer is that Nyx can have demigod children. However you have to wonder if she ever would have children with a mortal. Nyx is a very old goddess born from Chaos itself (much like Gaea or Erebus), so she was old even before humans were young. Not to mention her children spread darkness in humanity wherever they go, so Nyx probably doesn't care for humans and considering she lives in Tartarus probably doesn't help with mortal visitors. Not to mention her children would be so powerful that monsters would find and kill them before getting a chance. Just as a word of advice, I probably wouldn't want Nyx finding out someone is calling her a minor god.

As far as cabins, we don't really know how they figured out the current cabin system. Maybe they only built the current ones because those are the ones with children currently and so they get priority. There may be other demigod children that the gods haven't claimed yet, so they wouldn't have cabins yet.