A child's first trip on the subway is an awe-inspiring thing. This is especially true for little Barto, a Puerto Rican lad, for whom the whole city is wondrous. Rumblings of the strange underground world attract Barto as he peers into sidewalk gratings; and then sister Julie takes him on a trip to Grandma's via the Lexington Avenue IRT! Every aspect of the ride is graphically seen from this child's point of view and visualized for us through Sy Katzoff's excellent ""slice of life"" photographs. What provokes headaches in adults will evoke an opposite response in children as they travel with Barto on this thrilling underground excursion.