Black Hearts in Battersea by Joan Aiken

Simon is determined to become a painter when he grows up so he sets off to London to make his fortune. But the city is plagued by wolves and mysterious disappearances. The Twite household, where Simon is lodging, seems particularly shifty. Before he even gets a chance to open his glistening new paints Simon stumbles right into the center of a plot to kill the King. And worse than that Simon is kidnapped and sent to sea. Luckily there are two friendly stowaways aboard the feisty Dido Twite and the spoiled young Justin. But when the ship catches fire things look pretty dire. Can they escape? Will they save the king in time?