Swop! by Hilary McKay
Emily and Ben live with their wicked old Aunty Bess, who never lets them have any toys or sweets or pets. Luckily, Emily is fantastic at swopping things. She swops a song for a violin, her hair for a kitten, and even her grandmother for a donkeyl But will any of these things help them escape Aunty Bess for good? A charming and funny story - classic Hilary McKay!