As thousands of children are held at U.S. border, two...

Dozens of colorful butterflies sat still on the wooden patio table of a home in Alameda. About half a dozen stood motionless on a dining room table inside the house, while countless others rested inside a big, black garbage bag. More than 200 butterflies dangled on threads hanging from a rack outside the house, and countless others were packed in a cardboard box that was shipped from Kentucky.

The butterflies — made of recycled materials — may be overwhelming in number, and that’s kind of the point.

They sprang out of a project created by 11-year-old Kaia Marbin and 10-year-old Lillian Ellis, who began weekly origami meetups after Kaia heard a National Public Radio report in late June discussing migrant children being held at the border. Struck by the number of detained children, Kaia told her mother: “We should make something.”

In just a few months, Kaia and Lillian, with the help of friends and strangers, have launched a movement, the Butterfly Effect: Migration Is Beautiful, which now stretches from the Bay Area to different corners of the world. The girls created the project to raise awareness about, and stand in solidarity with, the thousands of children being held at Customs and Border Protection facilities.

“Butterflies (show) migration is beautiful,” Kaia said. “They all fly free.”

The girls have more than doubled their original goal of making 15,000 butterflies, and they’re now commanding the attention of elected officials. They plan to host a rally Saturday at Oakland’s Lake Merritt Amphitheater before flying to Washington, D.C., a few days later to speak with lawmakers and leave a display of 15,000 butterflies in the Senate Rotunda. They also plan to give 15,000 butterflies to Office of Refugee Resettlement officials who, they hope, will then take them to detention centers where the children they were made for can see them.

But that’s not the end.

“We want to just keep going until the kids are free,” Kaia said in a recent interview, as nine friends and several adults hung around the house folding butterflies, as they do every week.

Soon after committing to the project, which she and her mom agreed should be led by kids, Kaia sent an email to several close friends, including Lillian, asking to spread the word. They started telling classmates and meeting once a week in Alameda.

The children started with a goal of folding 15,000 butterflies by November, and weekends soon became “workends,” as Kaia and her “acta-buddy” Lillian, a compound for activist-buddy, call them.

“It’s really amazing,” Lillian said. “A lot of times we make 500 or more in just a short period of time.”

The girls and their parents started setting up tables at concerts and events to fold butterflies and recruit others to join the project. Their parents and other adults then circulated the idea on social media, which the girls are not allowed to use.

Word spread, and over the past few months, students in classrooms across the nation have been folding and shipping countless butterflies to Alameda, as have other families, hospital patients and lone supporters.

As of mid-November, more than 33,000 butterflies had been folded.

Several organizations have backed the project, such as Amnesty International, a human rights organization, as well as local groups like Destiny Arts Center in Oakland.

Kaia’s younger brother, 9-year-old Jahan, helped build a website that explains the project and displays an up-to-date count of origami butterflies, upcoming events, where to spot the butterflies and a guide to folding them.

There are butterflies on display at churches, schools and other spaces from Berkeley to Oakland to San Francisco to Queens, N.Y.

One of the more challenging aspects of the project — beyond finding a balance among school, friends and staying on top of a movement — has been dealing with people who don’t agree with the project, Kaia and Lillian said.

Some people have been more aggressive than others.

“Just people disagreeing and then hating on us and trying to scare us into silence,” Lillian said.

Kaia, sitting next to Lillian at a table full of butterflies on a recent evening, thoughtfully waited for her friend to finish.

“Mommy says that they wouldn’t be trying to scare us into silence if we aren’t actually making a big enough impact,” she said.

Kaia then paused for a moment.

“Something like that,” she said, before Lillian and her broke into giggles.

Sen. Jeff Merkley, D-Ore., doesn’t remember exactly where he learned of the butterfly project, perhaps while scrolling through social media or reading a news article, but he had one immediate reaction.

“This was so awesome,” said Merkley, who has been among congressional leaders trying to stop the separation of migrant families and the detainment of children.

“This is a way — a very powerful way — to draw attention,” he said in a recent phone interview. “It’s powerful and it is well done.”

With the help of Merkley’s staff, Kaia, Lillian and their parents plan to go to Washington, D.C., on Nov. 20, which marks Universal Day of the Child, and erect a display of 15,000 butterflies in the Russell Senate Office Building.

Merkley said he hopes to introduce the girls to other members of Congress, including Sens. Kamala Harris and Dianne Feinstein from California.

Before that, though, Kaia and Lillian plan to host Saturday’s rally at Lake Merritt to stand in solidarity with the children detained at the border. They said they will continue making butterflies until the children are freed.

“People say that children, like, can’t make a change,” Lillian said, emphasizing the last word. But she and Kaia have reflected on how much the project has grown in just a few months.

“They’re wrong,” the girls said in unison, interrupting each other.

Alejandro Serrano is a San Francisco Chronicle staff writer. Email: alejandro.serrano@sfchronicle.com Twitter: @serrano_alej