In his latest film, “I Wish,” the marvelous Japanese director Hirokazu Koreeda shows a strong affinity for the humors and longings of childhood. It’s an adult movie about children that feels made from the inside out. Sixth-grader Koichi (Koki Maeda) and his younger brother Ryunosuke (Ohshiro Maeda) have been split apart by their parents’ divorce – Koichi lives with his mother and grandparents in a southern region near an active volcano, while Ryunosuke lives with his layabout rock-musician father up north. It is the boys’ belief that a bullet train linking these two regions will become the agency of their salvation. The Japanese title for this film translates as “Miracle,” and that’s what the boys (real-life brothers) are expecting. They get it, though not quite in the way they expected.