The Whipping Boy, by Sid Fleischman, 1986

Jemmy-from-the-streets is a rat-catcher by birth and preference, but a royal whipping boy by occupation -- plucked from the sewers to live in the palace and take Prince Brat's countless whippings for him. When the prince grows bored of palace life and runs away, he makes Jemmy accompany him, but after they're captured by bandits, it's Jemmy's wit and resource that gets them out of one scrape after another and tests the prince's mettle. Tight, sharp, witty storytelling; strongly recommended.