I'm pretty sure everyone except the most hardcore "MUST HAVE BLOOD AND BOOBS!" audience (i.e. anyone 10-16) would agree that Nintendo isn't 100% childish. However, they almost always have some childish themes, except maybe in some of the Star Fox and F-Zero games. Not to mention, as has already been mentioned, try comparing them to Sony or Microsoft. Sorry, but even the most adult-oriented Nintendo franchises like Star Fox, F-Zero, Custom Robo, and Fire Emblem don't even touch Killzone, Halo, Gears of War, God of War, Uncharted, The Last of Us, and Infamous.
Of course, the very idea that Nintendo is bad because it is comparatively more childish is absolutely ridiculous. I can understand if you just don't like them, but the "they're too childish" accusation really isn't good at all. Consequently, I don't really think we need to defend their more family-friendly design strategies.