Exposing the children to both good and bad of the world, allow them to decide their own future. You don’t lay down answers, but make various paths for them to walk on. If we keep hiding what’s bad, they will be more intrigued by the bad, and begin taking good for granted.
A rational argument would always favour the good. So no need to hide the bad. Let it come in front and perish on its own. Keeping it in the dark will only give it time to strengthen.