Right read the initial post and the replies so far ,imho the e/mail/s were most definetly not written by a child .more likely the parent of one of them ,hence the completly load of b*****ks threat .as they wanted the original then through there toys out of the pram when you rightfully objected then I think got mummy or daddy who thinks there a barrack room lawyer involved .
If you have taken down the image or not ( I wouldn't ) I would send them a reply that if you find your images being used on the internet anywhere you will follow up with legal action over copyright theft .
That threat can be followed up I use pixsy.com for mine and have had some substantial payouts this year for image theft