Starcross, or, The coming of the Moobs! or, Our adventures in the fourth dimension! - Philip Reeve

Art and his family are invited on a fantastic free holiday to the exotic Asteroid Belt, in a remote part of space near Mars. Taking the train, they arrive to discover that nothing is quite as it seems - the hotel slips curiously back and forth through time, and the guests behave rather strangely too. What is behind these bizarre goings-on? It's up to Jack Havock, Art and his sister Myrtle (against her will) to get to the bottom of things. But the giant sand clams and man-eating starfish which roam freely nearby are nothing compared to the True Enemy, which is cunning, sinister, and almost unstoppable and may resemble a hat.