"Kappa have been used to warn children of the dangers lurking in rivers and lakes, as kappa have been often said to try to lure people to water and pull them in. Even today, signs warning about kappa appear by bodies of water in some Japanese towns and villages."
There are plenty of dangerous mythical characters too in Japan, ghosts, spirits and non-humans creatures, I am not sure where you got the idea that there's less than anywhere else in the world.
Just let's take Slavic folklore of Central/Eastern Europe with creatures like Baba Yaga (forest), Poludnitsa (summer fields, noon time heatstrokes), Rusalka (water), Vodnik (water) lot of fire and swamp daemons - everything which was considered harmful for kids.
These remains are still part of children education mostly with literature, rituals from pagan times and of course grand parents passing these stories.
It was a story about a demon that would hang in trees late at night, and if you were to walk under the tree the demon would drop down and latch onto your shoulders, and then eat you.
There was a mechanic in the mythology that involved telling stories with cliffhanger endings, or guessing the ending - the details are lost to me now.
All I know is that as a young child walking around at night, my pace would quicken when going under large trees. Those solemn rural nights, under the large jackfruit trees in the moonlight, the wind a steady whisper - you could almost get a glimpse of the demon's gleaming teeth in your mind's eye - inspiring a foreboding sort of solitude, a primal fear.
I do miss it at times.
The Japanese have like a pantheon of oni, ayakashi, etc. A lot of old Japanese art is honestly pretty freaky - almost completely opposite to the cute style that permeates Japan's pop culture today.
I can't remember where from, but I heard a fascinating take on how countries in Europe, with predatory animals and other dangers outside the village, would cultivate myths of things like werewolves, vampires, and other beasts that would kill/maul/bite you. This prevented children from wandering too far.
Meanwhile, in Japan (where there's very few predators to be afraid of), the myths are much more wholesome [1]. There's spirits that wash beans, lick oil, follow you around, and so forth, but few/no myths of the dangerous creatures you'd find documented in Europe.
[1] https://en.wikipedia.org/wiki/List_of_legendary_creatures_fr...