I couldn't disagree with your thesis more. Nintendo does not make games for kids. They make games for people who enjoy fun experiences. Nintendo has always been very successful at getting adults and children alike to love their games.
in response to your question...I don't think any game needs to be M, period. There are tasteful ways to achieve a T rating and still convey the same story, message, and gameplay. Don't get me wrong, I don't think there shouldn't be M rated games, but I don't think they are necessary...particularly from Nintendo.
Eh, i disagree. I don't think an M rating should be explicitly targeted, but sometimes 'tasteful' isn't enough. Half the fun of Django Unchained (a movie yes, but it applies to the story/message part of the discussion) is how it revels in its own violent absurdity. That quality is a big part of why people like Tarantino in general :p