Paper planes whizzed and dipped and careened into the audience while parents gleefully ducked for cover. At the front of the drab hotel conference room, a man deadpanned, “You all signed a waiver before you came in here?”

The speaker, Chris Cardiff, was urging the room of parents to take control over their children’s understanding of American history. His presentation raced through the “competing narratives” that young children faced: “celebrate ‘diversity,’” it went, versus “e pluribus unum.”

“Why would you have targeting of young children?” Mr. Cardiff said. “Because, first of all, people on the left side of the political spectrum are targeting preschoolers with their version of history.”

It was the start of a gathering, one of several run across the country by a group called the Great Homeschool Conventions, which brings together families who are part of America’s fast ballooning and politically forceful home-schooling movement.

In July, these families descended on a water park resort in Round Rock, Texas, where conference rooms were steps away from colorful mega-slides and plaster hippopotami. The children hit the lazy river while their parents attended sessions about designing bible-driven curricula and helping their offspring “safely navigate our hypersexual culture.” They discussed how to install patriotic values in their children and whether to send them off to college, where some speakers worried they would be exposed to left-wing values.

“At this point, I’m generally advocating people not going to college, unless you’re going into STEM,” one speaker, Lisa Nehring, told the room, later adding, “I sound like ‘Oh my gosh, the world is going to end’ — and it is!”