There are already many older teens and young adults who can't even wrap their heads around the concept of EV training; a child seeing EVs would get confused no doubt, and trying to explain EVs to them would probably turn them off from the game. That, of course, is every game developers worst nightmare. So they exist for the competitive players, but are kept hidden for the sake of people who don't want to deal with them.
The fact that in B2W2 Bianca, as the new "effort reader," tells you which stats your Pokemon are EV trained in is pretty significant in its own right. Also, it's easy enough to just mark off how many EVs your Pokemon has gained. It's pretty irritating when you're trying to reset EVs, but at the same time the berries stop working once they've hit 0, so even then it's not so much of an issue that they aren't shown.