The perception that Nintendo games are childish is wrong- they’re not childish as much as they are targeted at all ages, which just happens to include kids. In fact, in a lot of Nintendo games, adults will find some highly suggestive and provocative dialog, should they go looking.
But essentially, superficially, Nintendo games are kid friendly- there is no danger of playing a Nintendo game with gratuitous violence or expletive content, as an example, and you don’t have to worry about sexual content in a Nintendo game either (for that, you go look at fan art). Which is why The Legend of Zelda: Breath of the Wild‘s Japanese rating is so surprising- as spotted by NeoGAF, the game is rated for having sexual content, violence, and criminal activity. All of which is very… unlike Nintendo.
Now, to be fair, we already saw hints of this in the excellent story trailer for the game that Nintendo released– that trailer showed off large scale combat and conflict, as well as shot of Princess Zelda bathing. So it seems like this may just be a Zelda game that requires parents to use those parental controls on Switch that Nintendo have been talking up.