Seared into the memory of anyone who was alive to see it, the haunting visual of New York City’s mighty twin towers falling will never be forgotten.

But 18 years later, many young schoolchildren still don’t know what it’s all about.

That’s the inspiration behind Amy Schatz’s “What Happened on September 11,” premiering at 6 p.m. Wednesday on HBO. The filmmaker said she had to do it after a little girl in the third grade told her of a playdate where she and a friend Googled “Sept. 11 attacks.”

“When a child does that, what he or she finds are some pretty horrific images that are not necessarily appropriate for kids,” Schatz told the Associated Press. “So I felt a responsibility to try to fill that void and try to give kids something that isn’t horrifying and kind of fills in the gap.”

The resulting 30-minute documentary, aimed at kids 7 to 12 years old, is filled with easy-to-digest history lessons about NYC and the twin towers, from their construction to their devastating destruction.

“You can’t protect kids from what they’re going to come across,” Schatz said. “It seemed to me there was an opportunity to put something out there that is age-appropriate and not too scary and give them the tools they need to understand the world around them.”

The doc features Stephen Kern, who worked on the 62nd floor of the World Trade Center’s north tower, recounting the terrifying experience of being evacuated. Matthew Crawford, whose firefighter father died that day, shares the aftermath of family survival.

Schatz also visited a Secaucus, NJ, middle school that uses art and poetry to teach its students how to process emotions about the disturbing things they are learning.

The topic of Osama bin Laden is tackled — and Schatz doesn’t avoid some of the devastating imagery. Still, she opted not to dwell on them “so that we didn’t create too many lingering afterimages in people’s minds.”

“One of the biggest questions the kids have is ’Why? ‘Why would somebody do that? Why would there be such cruelty?’ ” Schatz told the AP. “That’s a very difficult thing to grapple with and answer so that was the trickiest part of the project.”

“What Happened on September 11” is followed by a companion piece at 9 p.m., “In the Shadow of the Towers: Stuyvesant High on 9/11” — which focuses on the memories of former students at Stuyvesant High School near Ground Zero.