Isso soa como "No Such Thing as a Witch", de Ruth Chew. Aqui está um trecho de seu resumo:
Nora and Tad are given fudge by a neighbor named Maggie Brown who has a lot of pets. After her brother and father start acting strangely, she deduces that one piece of fudge makes you like animals and that two lets you understand what they say. Mrs. Cooper gets angry when Tad and Mr. Cooper start feeding Skipper people food, and Nora goes over to complain to Maggie, who explains that the fudge wears off after a while. Nora and Tad visit Maggie again and pretend to each eat one piece of fudge. They each also take an extra piece. When they get it home, Tad eats two pieces, then three, starts acting a bit like a cat, then takes Nora's, and turns into a cat.
Você pode ler um pouco do livro no Google Livros aqui e veja se corresponde ao que você lembra.