Siri has been helpful for increasing my 3-year-old's vocabulary: he'll ask it to play a song he knows, and Siri will pick out an explicit R&B piece full of all sorts of words he's never been exposed to.</s>

(Seriously -- there's a song with the title "The ABC Song", which as one might expect, is about the ABCs. But it's impossible to get it to play that via voice control; it only wants to play "abcdefu", which is not the same song at all, and has a very clear F-bomb 5 seconds in.)