The Blue Child disappeared during patch 1.10.0 before the Burning Crusade. Anne's theory is that, since this coincides with Blizzard's addition of weather effects to the world, it was probably removed due to graphical errors. Whatever the reason, since then there have been only occasional references to its existence within the game: You could see it around the three-dimensional map of Azeroth in Halls of Lightning, and it was briefly visible during the Dragonwrath, Tarecgosa's Rest quest line. However, players were delighted to discover that it made a reappearance on the Mists of Pandaria beta, and it looks like patch 5.0.4 has finally restored the Blue Child to the world.
Thanks to Jeff Hoskins for the tip and Dawn Moore for the picture!