PHILADELPHIA — Patty Braxton grimaced as a priest led a few dozen mourners in prayer on a gray, drizzly Friday beside the grave of a small boy found dead in 1957 and long dubbed “America’s Unknown Child” or the “Boy in the Box.” Her father, retired Philadelphia Detective Thomas Joseph Augustine, worked the high-profile cold case most of his career. Both of the boy’s parents are dead, although he has living siblings, police said in announcing the slender 4-year-old’s identity last month.