George’s Marvellous Medicine by Roald Dahl.

A taste of her own medicine.
George is alone in the house with Grandma. The most horrid, grizzly old grandma ever. She needs something stronger than her usual medicine to cure her grouchiness. A special grandma medicine, a remedy for everything. And George knows just what to put into it. Grandma’s in for the surprise of her life—and so is George, when he sees the results of his mixture!