A six-year-old Missouri boy's mother, grandmother and aunt are accused of staging a kidnapping, then holding the child for four hours to teach him about stranger danger.

They hatched the plot to scare the boy because they said he was "too nice" to people, police said.

The boy's mother, Elizabeth Hupp; grandmother, Rose Brewer; and aunt, Denise Kroutil, engaged the help of Nathan Firoved, who worked with Kroutil at a petrol station.

Firoved waited for the boy to get off his bus after school, then lured the child into his pickup truck, police say.

While in the truck, Firoved, 23, told the boy he would never "see his mommy again," and he would be "nailed to the wall of a shed," police said.