Oh, that's just WRONG! HAHAHAHA... but what an enjoyable idea. How about... clenching the coin AND a small piece of chocolate? (without telling them of course...) Melting chocolate dripping down his/her leg? How NAUGHTY you've been! Punishment is automatic and unavoidable. Possibly whilst holding the coin to the wall? Oh, what a wonderful idea, thank you all so much!
monsignor