Everybody knows that childhood in America should be carefree. A time to chase fireflies, fish trout, and play kick the can before supper. These days, though, growing up in America is much more dangerous – a time to soak up fast-food advertisements, internalize the violence of video games, and dodge the bullets of trigger-happy classmates. What happened to that idyllic American childhood?

It never existed. “We cling to a fantasy that once upon a time childhood and youth were years of carefree adventure,” writes Steven Mintz in his fascinating new book, “Huck’s Raft” (Belknap Press, 464 pages, $29.95), which examines the lives of young Americans from colonial times through the present. “For most young people in the past, growing up was anything but easy.”

To dispel our fantasies, he unleashes a frightening onslaught of unhappy children from the underbelly of American history. Children who work in coal mines. Orphans who beg for dinner. Babies born into bondage. Mr. Mintz draws upon a vast range of sources, including personal diaries, novels, sociological studies, family memoirs, films, and newspaper accounts. Along the way, he pops in on every corner of the country’s childhood history from the orphan trains of the 1860s to the schoolyard shootings of the 1990s.

Mr. Mintz launches this mission from the banks of the Mississippi River, where he revisits Hannibal, Mo., the original stomping grounds of young Samuel Clemens, later Mark Twain. Mr. Mintz shows that the real-life Hannibal bore little resemblance to the romantic river settings traversed by Twain’s famous literary creations, but his real purpose is to establish a leaky metaphor. In a clumsy bit of raft-jacking, he tries to steer Twain’s literary device to more somber shores. Huck’s raft suggests, he says, the “physical, psychological, emotional, and socioeconomic challenges of childhood.”

He goes on to divide American youth into three overlapping eras: premodern, modern, and postmodern childhood. None of which were overwhelmingly blissful.

Pre-modern parents, such as the Puritans of New England, gazed at their newborn babies and saw sinful creatures capable of sloth, jealousy, and deception. Colonial parents therefore spent a lot of time hustling their youngsters toward adulthood. They were corralled into various soul saving activities, such as home schooling, church, apprenticeships, and more church. There was little to savor.

The more romantic vision of childhood that swept the country in the middle of the 18th-century, Mr. Mintz explains, coincided with the rise of the middle class family. These parents considered their babies pure, and they strove to protect them from by the adult world for as long as possible. Children began spending more years in school. Childhood labor was outlawed. The peer group became all-important. Children were granted autonomy from adults.

Sometime in the 1950s or 1960s, however, Americans shifted from protecting children against the adult world to preparing them for it. In today’s postmodern era, children’s naivete is no longer considered admirable, writes Mr. Mintz. It’s considered dangerous. Parents once again herd their offspring into adult activities at younger ages. Boys attend pre-professional summer camps. Girls spend Saturday mornings studying for the SATs. Children learn about AIDS at school and schedule playtime by appointment.

Throughout, Mr. Mintz emphasizes that people tend to glorify past eras of childhood in order to lament the current one. “Ever since the Pilgrims departed for Plymouth in 1620,” Mr. Mintz writes, “Americans have experienced repeated panics over the younger generation. One of the goals of this book is to strip away the myths, misconceptions, and nostalgia that contribute to this pessimism about the young.”

In the end, Mr. Mintz takes on countless American kids from Sybil Luddington (the so-called “female Paul Revere”), to Baby Jessica, to JonBenet Ramsey. He dedicates a chapter to children born into slavery and another to immigrant children. He writes about children who fought in the Civil War and those who lived through the Great Depression.

His book descends, at times, into tedious minutiae. But he successfully steers every runaway tangent back to his broader themes. His writing is straightforward. His examples are enchanting. His arguments are convincing. And wherever he takes the story of American childhood, he’s worth following.

Mr. Gillette last wrote for these pages on the role of dogs in American history.