It has come to my attention that none of the forbidden kids should be forbidden. The Great Prophecy speaks of a half-blood (demigod) of the eldest gods.
None of the Big Three are the eldest gods. Yes, they are the oldest Olympians but oldest Olympians does not equal oldest gods.
Assuming having demigod children is a requirement for this prophecy, we already have one option for a godly parent who should’ve been forbidden to have mortal offspring, Hecate.
Hecate is the goddess of witchcraft, and the Mist in the Riordanverse. We know she has mortal children thanks to her cabin and the short story, Son of Magic. Not only that but we were introduced to her cabin’s head councillor, Lou Ellen who we meet in Lost Hero, I assume.
I’m inclined to say at least two of the Big Three still count unless there are two other gods older than the Big Three that have half-bloods I do not know about in the Riordanverse.
If you know any, please leave them in the comments. We will build this AU together.