The Hidden Staircase by Holly Webb
At Penhallow Hall, where magical objects come to life, Polly and ghost hound Rex discover a toy dog on the nursery shelf. But to their surprise, this dog doesn't want to be woken! Polly finds out that the little toy dog was left behind at Penhallow during World War II by his evacuee owners, when the two children returned to the city. Polly invites her new friend at school, Lucy, over to tea and she seems strangely drawn to the toy dog, too. But before Polly can find out why, Lucy goes missing. Polly guesses where Lucy might be - but she'll need the little dog's help to find her.