The Magic Coin by Ruth Chew

The Magic Coin by Ruth Chew was first published in 1983, and is a magical realism story about  a young girl who is given a magical coin as change when buying the newspaper for her dad from a newsstand one morning. She and her brother, Christopher, soon discover that the coin has magical qualities. The smiling man, who winks from one side of the coin, can assist them in all kinds of ways. The coin can morph into all manner of things, and also makes difficult fraction homework seem easy. One day, after some bullies toss the coin into a lake in Prospect Park, the siblings find themselves transported back into the times of barrels, swords pistols, longboats, and sailing ships. They find themselves in the middle of a sea battle between a  British merchant ship and a Spanish pirate ship. Can the powers of the coin return them to their own times?  Chew also did the soft pencil illustrations which add greatly to the story line.

Christopher and Meredith find themselves in another century