Lulu and the Hamster in the Night — Hilary McKay

When Lulu goes for a sleepover at her nan's house, she isn't supposed to take her new hamster. But how can she leave him behind? Lulu adopts a hamster, but she can't tell Nan about it because Nan doesn't care for most animals, especially hamsters. It's her birthday this weekend and Lulu and Mellie are staying the night. Lulu needs to bring her new hamster with her, but can she and Mellie keep it secret from Nan over the weekend?