Showing Gracie Fairshaw and the Mysterious Guest by Susan Brownrigg

Gracie Fairshaw and her family have barely moved into The Majestic, a Blackpool boarding house when Ma mysteriously vanishes. She teams up with her younger brother George, and befriends siblings Violet and Tom, and maid Phyllis. They must work out why one of the guests - a conman conjurer - has made Ma disappear!