The Tunnel of Hugsy Goode — Eleanor Estes

Nobody believed Hugsy Goode when he prophesied that a tunnel lay beneath the alley until--generations later--Nicholas (alias Copin) and Timothy (alias Tornid) decide to explore. And lo and behold, right under the vine-covered hole outside the house where Hugsy Goode used to live, they find an entrance to adventures beyond their wildest dreams.
A sequel to The Alley.