Fictional though they are, Tom Sawyer and Huckleberry Finn exemplify a lost truth about American kids up until the 1880s: Childhood was wild .  Like millions of their real-life contemporaries, the protagonists of Mark Twain’s comic masterpieces roamed unsupervised across town and country, looking for fun and adventure, often finding trouble and danger. Then came the Progressive Era and one of modern America’s defining inventions: the playground.  Playgrounds helped transform childhood from participation in public life into preparation for adulthood. From now on, childhood would be supervised and sanitized, zoned into a designated area and limited to a sandbox. No more pirate play on the Mississippi — for better or worse.  The end of the wild childhood While not every kid explored caves, roamed graveyards at night, or faked their own deaths like Tom and Huck, the average American childhood during their era was largely unsupervised, even in the big cities.…