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Unless the child goes through with the act, it remains a case of Blackmail.
Edited by eroock
The threat if enacted would be a Wounded Gazelle Gambit- pretending to be in danger to sic a third party on your foe.
Compare and contrast Playing the Victim Card where Alice does this to make Bob look like a villain for stopping her villainy, and False Innocence Trick where Bob the demon claims to be a poor innocent fairy who will reward Alice if she frees him from the mirror he's imprisoned in.
Edited by Scorpion451
A child blackmails an adult by threatening to call the police and claiming to have been kidnapped