The Boy with the Magic Numbers by Sally Gardner
When Billy Pickles' dad leaves home to live in New York, he gives Billy a moneybox. Billy's not sure where to put in the money, and not sure why his dad has left.
Then Billy goes to New York to visit his father and his Italian grandmother, and discovers the secret of his magic moneybox. In a wonderful adventure, he takes Mighty Mamma on a helicopter ride, cracks open a safe, solves a kidnap mystery - and gets to know his dad again.