Let's clear this up. MoP is not a "childish" expansion to any higher degree than the game itself is childish. Because World of Warcraft is childish (by the general criteria of people complaining about how childish MoP is supposed to be). It always has been. It always will be.
Remember when MoP was announced? Some people got their internets all in a bunch about some sudden childish tone the game was taking. Know what else was going on when MoP was announced? Hallow's End. Know what you do there? Get some candy, run around collecting masks, and go drop some stink-bombs on an enemy city as a prank. How many of you stormed the forums to complain about childishness of Pandaren and then logged in to do your stinky daily? Be honest.
Remember the Battle for the Undercity in WotLK? Of course you do, if you were playing then. A big long epic battle through the Undercity alongside your faction leaders. Not childish. If you were Alliance, likely after that you headed over to Grizzly Hills for some more questing. Once there, probably your first quest was eating some nuts you're not supposed to, so you've got to give yourself a laxative and go poop 'em out quick. Yup, that's a nice change of gears. Epic battle! Epic pooping! Slightly childish. Slightly.
Think that's the only quest that involves digging through poop? If you've quested everywhere you know the answer is no. Not by a long shot. How childish.
Speaking of Hallow's End, how about every other holiday event in the game, ever. It's all childish (Except for maybe Children's Week. School of Hard Knocks indeed, that ain't for no children.)
But Pandaren are a whole RACE of childishness!!! you say. Well no, given what we've all seen of them by this point, but I'll pretend you're right anyway. How absurd then. They won't fit in with all the srs biznes races of Azeroth. Like Goblins. Or Gnomes. Races who have never had any whimsy associated with them.
So yes! Let us complain about how childish these Mists of Pandaria are! Then let us go to Redridge, and play an absurdly overdone Rambo questline! And Azshara, where it is grim and humourless! And Silverpine, where the invasion hinges on all the Orcs not being so totally wasted man!
ONWARD TO NON-CHILDISHNESS!!!
(Author's note: This post is meant to be read with a level of sarcasm. I am not trying to flame on WoW. I love WoW, I am excited for MoP. I just think it's rather silly to suddenly be all up in arms about how "childish" MoP is supposed to be when that sort of light-hearted, jokey, pop-culture, low humor content has been in WoW since launch. The thread title is meant as an observation, not a complaint.)