"Naiveté," we say. They'll be disabused of that notion soon enough. They'll see the world for what it is, and how hard it is to change.
But what if we let them keep their naiveté?
What if we were already busy changing the world? What if we brought all of our experience and wisdom and cynicism to bear on the task at hand? What if instead of telling them they can't do it we invited them to be a part of what we're already doing?
What if we taught them what we know and unleashed their boundless energy to go about the task?
What if we let them change the world?