when i was growing up, no one told me about tragedy and malevolence – not parents, school, TV, books. (fantasy books do present evil, but outside the context of the real world, as a fantasy.) so i was not prepared to face the human condition (which has positive aspects, potential and opportunity, but there's also plenty of weakness, sin, suffering, etc. the world is full of problems, some quite hard). i was told society is great, evil is rare and weak, reason and success are common.
it's been difficult finding out how fucked up the world is. it would have been easier to face from the outset instead of as a readjustment later.
One of the meanings here is: don't lie to children about the nature of the world to shelter them in certain ways (other ways of protecting children are good, and the distinction takes some serious thought to get right). but i don't think that's primarily it. i'm from the San Francisco Bay Area. neither my parents nor teachers knew what the world was like themselves!
I agree that fiction & TV often make evil seem unreal and a fantasy. Maybe because the evil is all represented by one main character. There are imo some children's books that do a great job of describing the pervasive and hard to spot nature of evil. e.g. Lemony Snicket's 'A Series of Unfortunate Events'. Even though it starts with a traditional representation in vol. 1. The following books (there are 13) go on to unpick this.