Wilkins' Tooth — Diana Wynne Jones

OWN BACK LIMITEDREVENGE ARRANGED, PRICE ACCORDING TO TASK, ALL DIFFICULT TASKS UNDERTAKEN, TREASURE HUNTED, ETC. It seemed like a marvellous scheme at the time! Frank and Jess had set up the business because they needed money - their pocket-money had been stopped just when Frank owed Buster Knell ten pence. Their father had put a quick stop to ERRANDS RUN, so why not try something that was bad instead? Offering to do dreadful things that other people didn't dare to do? So they pinned up the notice on the potting-shed and waited for customers. But of course, nothing's ever THAT straightforward.