WASHINGTON (WASHINGTON POST) - In the middle of a sit-in at the Hart Senate Office Building, protesters began to make demands.

They wanted crayons. A snack. That action figure that turns into another action figure.

On Thursday (July 26), it was the children who led the charge.

About 100 people, many of them very young, marched, toddled and, in some cases, were carried from St Mark's Episcopal Church in Washington, DC, to the building where about 50 senators have offices.

When they arrived, they gathered in a circle, hoisting signs that declared, "I AM A CHILD". They sang songs that riffed on familiar nursery rhymes - "If you're powerful and you know it, clap your hands" - and cheered, stomped and danced.

Thursday was the Trump administration's court-imposed deadline to reunite thousands of children and parents separated by immigration agents at the United States-Mexico border. Those younger than five were supposed to be returned to their families earlier this month, although the administration did not reunite all of them by the court-assigned date.

Hundreds of children whose parents have been deemed ineligible by the government will remain separated, including those whose parents were deported without them.

The government was expected to update a federal judge on its progress by the end of the day on Thursday.

Protest organisers said they wanted to bring children front and centre so lawmakers could see the kinds of kids the president's "zero tolerance" policy has affected.

"This is what a two-month-old looks like," said Ms Jenn Kauffman, 38, gesturing to the wide-eyed baby affixed to her chest. "He's still breastfeeding. He depends on me for nourishment, for everything. I don't know what either of us would do if we were separated."

Parents at the rally said they wanted to show their children what it means to stand up for others in the face of injustice.

"Do you remember what we talked about, what it means to be here at this protest?" Ms Margo Simon, 41, prompted her son. "It means equal rights," said Indy, four, pumping his fist into the air.

Nearby, a three-year-old girl flashed a metallic red cape emblazoned with the Wonder Woman symbol. "When I told her we were going to go to protest and try to help those children, she said, 'If we're going to be superheroes, I need my cape'," said Ms Diana Raverlie, 44.

Senator Kamala Harris asked the children to show her drawings they had made of their families and the messages they had written.

Suri Baez, seven, had drawn two hearts - one broken and one intact. The broken one is for families who were separated, she told Senator Harris. Their hearts will be whole again when they find their way back together.

"Oh, that's fantastic," Senator Harris said, crouching down to look up at the girl. "So there was a broken heart and now there's a good, solid heart."

Suri nodded.

"It is the ultimate act of inhumanity to rip those children from those parents. And to do it for what? For the sake of so-called deterrence?" Senator Harris told the crowd. "It is important that we all stand up and say, 'We as a country are better than this'."

The protest, organised by a coalition of groups calling themselves Families Belong Together, echoed a demonstration at the same building in late June in which nearly 600 protesters were arrested.

On Thursday,by the time Capitol Police issued warnings to disperse, the children had drawn the attention of Senate staff and tour group members, who gathered at windows and craned their heads to watch.

"We don't want to arrest anyone who doesn't want to be arrested today," an officer told one of the organisers as parents helped children tuck their signs away.

Suri was there with her two siblings, Roberto, 12, and Luna, 14. For the three siblings, who came to Washington, DC, from Denver to participate in the march, the memory of their mother, Ms Jeanette Vizguerra, being removed from their home was still fresh.

Ms Vizguerra, 46, an undocumented immigrant from Mexico City who has lived in the US for 21 years, was detained last year for 86 days.

Immigration officials picked her up for violations that included driving without a valid licence and crossing the border illegally, she said. Her children are US citizens.

"I feel like it's kind of similar, us being without our mum and those kids being without theirs - it's cruel and inhumane," Roberto said. "I know I'm not going through the same pain that those kids (at the border) are, but I wanted to be here so I could feel like maybe we're helping them."