The show boss offered to take part after being impressed by the 11-year-old contestant who showed up to the heat in Birmingham at the weekend.
Panellist Cowell nervously stepped up on stage as the youngster's assistant after being encouraged by the audience, standing against a wooden board ready to face the airborne blades.
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But while the first knife thrown had plenty of clearance from the multi-millionaire's body, the second whistled perilously close to his shoulder and bounced off the board, clattering on to the floor.
Cowell opted to retire while he was still in one piece, shaking the boy's hand.
Fellow panellist Amanda Holden told the young hopeful called Edward: "You nearly killed Simon". And fellow judge David Walliams added: "How much would I have to pay you to throw the knife a little bit nearer?"