PHILADELPHIA — DINNER with your children in 19th-century America often required some self-control. Berry stains in your daughter’s hair? Good for her. Raccoon bites running up your boy’s arms? Bet he had an interesting day. As this year’s summer vacation begins, many parents contemplate how to rein in their kids. But there was a time when Americans pushed in the opposite direction, preserved in Mark Twain’s cat-swinging scamps. Parents back then encouraged kids to get some wildness out of their system, to express the republic’s revolutionary values. American children of the 19th century had a reputation. Returning British visitors reported...