Witch's Business by Diana Wynne Jones

They're in the revenge business!

Jess and Frank's father has stopped their allowances for four whole months! That means that Jess can't go anywhere or do anything with her friends. Worse yet, Frank owes money to Buster Knell, the bully. How can Jess and Frank earn some cash—fast?

By starting a business, Own Back, Ltd. It specializes in revenge, which every kid needs to seek at some time, they figure. Most don't have the courage themselves. But Jess and Frank do—for a price!

Lots of clients show up. But Jess and Frank soon discover that the revenge business can be pretty complicated, especially when it turns out that there's another one in town—owned by Biddy Iremonger, the fiercely competitive local witch!