I read this short story at least 30 years ago, maybe more. I cannot remember at all where, but probably a collection.

On a planet where humans have a some settlements, there is a very dangerous monster, but there is a way to protect children from it, provided they are not too small, and they protect in turn the entire human settlement.

The monster is telepathic. Very small chidren are told about this horrible creature. But when they grow a bit older they are told that, no, of course, such a creature does not exist, it is just something babies are told about, like Santa Claus.

So in this age group, they are safe, because when they see something approaching that looks like what they were told earlier, they think "No, this is impossible, there is no such creature, it is just a bogey for babies" and the telepathic monster receives this thought and collapses. It recovers eventually but the experience is traumatic and it learns to avoid areas where humans live. Eventually adults learn that the monster really does exist, but there is always enough children in the right age range to protect human settlements.



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