Really, though, while that's the most logical conclusion, we're never told what it truly is. What would the KoW want with a child anyways?
http://en.wikipedia.org/wiki/Rumpelstiltskin
From the Biography of Barenziah we know that elves generally have very few children and the firstborn (biblically many times the firstborn actually means firstborn son, I'm assuming this is the case with the KOW) is the heir to the throne of Firsthold since we know the current King of Firsthold's son is dead. The KOW raises the kid, teaching him everything he needs to know, then inserts him into the court of Firsthold when necessary to act as http://en.wikipedia.org/wiki/Mordred would.
It is a simple thing for the KOW to let the King of Firsthold talk to his dead son, and it shouldn't be difficult to raise the elven child, so this is almost a no-risk high-payoff gain for the king if it goes well. And it shows off the personality of Morgiah extremely well, what lengths she is willing to go to to get things done.