Photo: Courtesy of Olena Blumberg For an idealized image of homeschooling — a glimpse of the life parents and children might lead off the clock, out of institutions, on their own — one could do worse than visit Heckscher Playground on the south end of Central Park early on a Wednesday afternoon. Most of the kids in New York are still in their classrooms, but a sizable group of children meets here weekly. Kids arrive with their parents, holding hands or racing ahead. Some are neatly dressed,