Sir, I am deeply offended.
Nintendo has as much hardcore as MS and Sony do, or even more. Look at Monster Hunter Tri. Look at Sin and Punishment or Kid Icarus: Uprising. There are some immensely challenging games on the Wii, and the DS even moreso. I'm mostly a Nintendo gamer but I'm hardcore and I get most of my fill of hardcore gaming right on my "kiddie" (not your words, I know) systems. The hardest game I can remember playing lately on PS360 was Dark Souls, which isn't THAT tough. I regularly die more often in Nintendo games, and they can be just as punishing about it, if not MUCH moreso.
I don't think you understand what hardcore means if you think only Sony and MS are hardcore. Hardcore is about loving difficulty, and it's about loving video games of all kinds. If you're hardcore you don't scoff at a game and turn the other way; you look at all games, open-minded to any experience that can be fun. Hardcore doesn't mean wanting nothing BUT a challenge in a game, but it means you enjoy being pushed to (and beyond) your limits and improving as a gamer. It means you look at the difficulty settings your first time through and immediately choose the hardest one. But it also means you really look into other consoles and genres that you think you might not like. If someone says a game is good you don't necessarily rush out to buy it, but you don't shoot it down because it's not to your initial liking.
Little King's Story was one of the tougher games I've played this generation. Super Mario Galaxy, despite its relatively easy difficulty level, was one of the most fun. Zack and Wiki had the most difficult puzzling I've encountered this generation. Sin and Punishment had me looking at more game over screens in a 4-hour play session than I've seen in the entirety of Dark Souls. Monster Hunter Tri had more of the most challenging and demanding combat I've enjoyed in an action game. (unlike Dark Souls where a LOT of deaths felt more like the game's fault then my own)
The Wii is very much overlooked when it comes to hardcore games. In general, it's just plain overlooked by everyone. Most people who bought it got the popular first-party titles and ignored the rest. Sorry for the wall of text and I'm sure you didn't mean to say what you did, but hardcore and mature are NOT the same thing.
On that note, it's irrelevant. If Nintendo's Wii U manages to be powerful enough to compete even somewhat with the next-gen consoles, it should be able to handle almost every multi-console title. By the sound of it, it's an easy platform to develop for and Nintendo is listening to developer input on it and changing things accordingly. So I doubt that 3rd party developers will skip Nintendo for their PS4/720/PC titles in the new generation. Thus, the whole "only kiddie games" idea will fly out the window.