The Toy Brother — William Steig

Yorick Bede has always considered his younger brother Charles a first-rate pain in the pants, and Charles thinks the same of Yorick. One day Yorick plays with his alchemist father's potions, and he accidentally shrinks himself to the size of a cockroach. Now suddenly Charles is the big brother. Though it's tempting to leave Yorick as a shrunken version of himself to teach him a lesson, Charles and the rest of the Bede family protect Yorick and tinker with more and more potions until they find the right one to bring back him back to his normal size.