They’re treating it like a national playground.

At the National September 11th Memorial, tourists balance coffee cups and soda bottles on the parapets bearing the names of the dead.

Parents hoist their children to sit on the bronze plaques, while other visitors splash water from the two waterfalls onto their faces to cool themselves on a hot summer day.

On the plaza, tourists break out lunch foods and lie on their backs.

A year after the memorial’s opening, the almost-cheerful atmosphere at what was supposed to be a solemn site has appalled first responders and victims’ families.

Some have compared the $700 million memorial to a “Disney attraction,” down to the weaving lines to get in.

One tourist “spilt coffee all over my son’s name . . . after she arose from sitting on the names,” a relative wrote to Bill Doyle of the Coalition of 9/11 Families.

When first responder Marianne Pizzitola visited, she found people acting “like this was a park or playground.”

“People laughed and took pictures smiling, and so many people leaned on the tablets with all of my friends names engraved in them, holding Starbucks cups, like it was a kitchen table,” Pizzitola, head of the FDNY EMS Retirees Association, wrote in a letter to Memorial President Joe Daniels.

Last week, The Post observed guards circling the two pools and prohibiting visitors from leaning on the ledges or resting their bags against engraved names.

Two guards said the crackdown was new, a directive issued because of the upcoming 9/11 anniversary, to prevent scratches or damage to the monument.

The relaxed atmosphere at the 9/11 Memorial stands in contrast to other national memorials. At the USS Arizona Memorial at Pearl Harbor, visitors are shown a video explaining the significance of the site before entering. A large sign at the entrance reads: “Please conduct yourself with dignity and respect at all times. Remember this is hallowed ground.”

At the 9/11 Memorial, a list of visitor rules directs them to a Web site for more information about respecting the memorial. One sign on the plaza reminds visitors that the memorial is “a place of remembrance and quiet reflection.”

At least one family member is relieved the site’s mood isn’t morose.

“I have always assumed that as time goes by, people will come there for gentle recreation — walking, sitting in the dappled sunshine, even picnicking — so I am rather surprised that anyone thinks this is not acceptable behavior,” said Kathy Bowden, who lost her brother in the attacks.

Said etiquette expert Jacqueline Whitmore, “When you step upon a memorial site, you should have some respect, keep your voice low, and remember it’s not an amusement park.”