In Vanilla, all we really had that was really kiddish was Gnomes, and at least they were contrasted by those irradiated guys that forced them out of their city.
In BC we had Blood Elves, which were pretty damn childish (even the way the males stand looks ridiculous), but at least we were still off killing Illidan, and fighting Demons
Along comes Wrath, and we get zombie Warriors, a vastly easier game, and schoolyard games such as ToC 5 jousting, and while ToC 5 jousting was by and far the most child-appealing thing to date in the game, we at least had Arthas as the light at the end of the tunnel, a refreshing throwback to the similar character named "Arthas" from a much older game made by some of the same people called "Warcraft 3"
Cataclysm was a near-breaking point, though, all previous issues pale in comparison to the Worgen race. A race clearly designed to pander the the facebook generation, and fans of the Twilight series. This, however, was not the end; far more childish than even the Worgen, was the GOBLIN race. A race of child-appealing near-humans was discolored skins, and "funny" themes and dialogue, male Goblin NPCs even have a dialogue reference to Jersey Shore.
Mists of Pandaria goes without saying; I didn't make this thread to complain about it, so I won't rant on about how childish it is; but even though it goes without saying...
MoP is the most childish thing I've seen come from Blizzard Entertainment, or Related to World of Warcraft, to date