Kids peer into a hole in the wall of a barn.Photo: Marc Fader of City Limits The chartered bus pulls up to the 100-acre Hudson Valley farm, after a two-hour drive from Harlem, and stops alongside a row of crates filled with red and yellow onions. Outside the bus, sprawling green fields have come into view. "Oh my God. That's a barn," declares a young girl on the bus, her gaze fixed on the two-story building that has appeared in the distance. "I've never been in a barn." The 52 people on the bus are employees and patrons of seven …