MATAMOROS, Mexico — In the middle of the largest refugee camp on the U.S. border — close enough to Texas that migrants can see an American flag hovering across the Rio Grande — Marili’s children had fallen ill.

Josue was 5. Madeline was 3. The small family was huddled together in a nylon camping tent with two blankets last week when the temperature sank to 37 degrees. The children started coughing, Marili said. Then their fingers and toes turned bright red. The camp’s doctor had begun to see cases of frostbite.

Like most of the roughly 1,600 asylum seekers at the informal camp, Marili and her children had crossed the border into the United States this summer only to be sent back to Mexico to await their asylum cases — part of a year-old U.S. policy called the Migrant Protection Protocols.

In recent weeks, dozens of parents have watched as their children, sleeping outside in the cold, have become sick or despondent. Many decided to get them help the only way they knew how — sending them across the border alone. As Josue and Madeline grew sicker, it was Marili’s turn to make a decision.

These cases illustrate the human toll of the Trump administration’s policy and suggest the United States, Mexico and the United Nations were unprepared to handle many of the unforeseen consequences.

Marili, fleeing gang violence in Honduras, knew that unaccompanied children were admitted into the United States without enduring the MPP bureaucracy and the months-long wait. The 29-year-old mother — who, like others here, asked not to be identified by her last name, for fear it could affect her asylum case — believed that returning home would be suicide. So she bundled up her children in all of their donated winter clothes and scrawled a letter to U.S. immigration officials on a torn piece of paper.

“My children are very sick and exposed to many risks in Mexico,” she wrote. “I don’t have any other way to get them to safety.”

She pressed the letter into Josue’s hand, she said, and pointed the children to three U.S. Customs and Border Protection agents in the middle of the Gateway International Bridge, the span across the Rio Grande that connects Matamoros to Brownsville, Tex.

“Josue told me, ‘Please don’t send us,’ ” Marili said, crying at the memory. “But as a mother, I knew it was the best decision for them.”

Then she sprinted to the bottom of the bridge and watched through the fence as her children turned themselves in, weeping and wondering when she would see them again, hoping they would find their way to her husband. He had entered the United States and applied for asylum before MPP was implemented. He was allowed to stay.

In the past three weeks, migrants and aid workers say, at least 50 children have made the same crossing. The Washington Post interviewed the parents of 20 of them. On Tuesday morning, three more children were sent over. On Wednesday, another three. From tent to tent, families now talk openly about whether and when they will send their children.

More than 47,000 migrants have been sent back to Mexico since MPP started in January. Through September, 9,974 cases had been completed; only 11 migrants, or 0.1 percent, had received asylum, according to the Transactional Records Access Clearinghouse, or TRAC, a research center at Syracuse University.

“It’s becoming clear to us that this whole thing is a lie,” said Reyna, 38, who sent her 15-year-old daughter, Yoisie, across the border last week. “They tell us to wait and wait and wait, but no one here gets asylum.”

The Department of Homeland Security did not return calls seeking comment.

Asylum seekers began sleeping out in the wooded field here at the base of the international bridge in August. They receive no assistance from the United States or the United Nations. They rely instead on tents, clothing and food donated by a group of American retirees and medical attention from a nonprofit group whose one doctor sits under a blue tarp.

U.N. officials say they were told months ago that the migrants would be moved by the Mexican government to better conditions. It hasn’t happened.

“We started hearing about the situation, but we just didn’t have enough capacity to help,” said Dora Giusti, the head of child protection at UNICEF in Mexico. “And the Mexican government kept saying [the migrants] would be moved out of the state, so we were waiting to see if we could respond there.”

The U.N. refu­gee agency says border cities in Tamaulipas state, where Matamoros is located, “are among the most insecure and dangerous in the country, which has limited our actions on the ground.”

The municipal government opened a shelter at an indoor basketball court last month. With a capacity of 300, it’s already full. It’s also miles from the bridge, making it more difficult for migrants to reach the border for their court dates, or to meet with pro-bono lawyers. Every day, the U.S. government sends dozens of migrants to Matamoros under MPP. They are taken directly to the encampment and often sleep outside until they find a tent.

The camp consists of hundreds of tents clustered together on a spit of sidewalk and a stretch of scrubland along the Rio Grande. There are only a few showers, so many people bathe and wash their clothes in the river. Once a dead cow floated by and became lodged next to the camp. Another time, the headless corpse of a man washed ashore.

A cold front settled here for three days last week. Immediately, children started getting sick.

Gabrielle, 15 — from San Pedro Sula, Honduras — started coughing. Sarai, 12 — also from Honduras, from Santa Rosa de Copan — was vomiting. Valeria, 5 — from the Honduran capital, Tegucigalpa — developed a fever and became despondent.

Global Response Management, the Florida-based nonprofit that runs the small medical clinic under the blue tarp, saw a surge in patients, most of them children. The most common cases were respiratory illnesses, said Megan Algeo, the doctor on call at the time. In one case, Algeo said, she persuaded U.S. immigration agents to admit a child for emergency care.

Parents in different parts of the camp decided it wasn’t fair to keep their children here. Some joined a Facebook group to discuss their options and what would happen if their children crossed the border alone.

“I kept thinking, my daughter is going to die here,” said Blanca, Valeria’s mother.

They all had relatives in the United States. Their idea was to send their children to live with spouses, siblings, cousins while they waited in Matamoros to complete the asylum process. They worried about another cold front, or another flood (there was one in September), or cartel-sponsored kidnappings.

Gabrielle walked across the bridge alone, carrying a plastic bag with her asylum papers. Sarai went with a friend. Valeria and her sister, Anahi, 7, crossed together, holding hands.

All are now in shelters in different parts of the United States. Under U.S. policy, children who enter the country unaccompanied are taken into government custody until authorities can connect them with relatives to whom they can be released.

Glady Cañas, who runs Helping Them Triumph, one of the few humanitarian organizations at the camp, tries to persuade parents not to send their children alone.

“Why did you send your child?” she demanded of Israel, Gabrielle’s father.

Israel, 40, stared at the ground. They were standing in front of his blue tent.

“She was sick,” he said. “We were desperate. A child can’t wait here for a year like this.”

Cañas hugged him.

“I personally don’t agree with what they are doing,” she said later. “A child needs their parents. But when you look around here, you understand the desperation.”

For many families here, the children — and the threats against them — were the reason they fled their countries in the first place.

Victor, 28, left El Salvador with his daughter, Arleth, now 10, after she was sexually assaulted by a man affiliated with a local gang. Victor pressed charges. He carries court documents and hospital records that substantiate the case in alarming detail. The man was sentenced to 12 years in prison for “sexual aggression of a minor,” one court transcript says.

As soon as he was sentenced, Victor said, gang members came after the family. In August, they fled.

Victor and Arleth were sent back to Matamoros on Aug. 28, before tents were available. They spent 15 days sleeping outside. Eventually, he found a job in a Chinese restaurant earning $7 per day. He saved up and bought a camping tent.

But after two months, Arleth was sick, vomiting all the time. Their tent had flooded twice in the rain. After her assault, she struggled to remain calm in large groups of people, and she hated walking across the camp to use one of the portable toilets.

Victor took her several times to the Doctors Without Borders nurse who came to the camp twice a week. But she never improved.

In late September, on Arleth’s 10th birthday, Victor bought her a cake and five candles. He asked someone in a neighboring tent to take a picture of them smiling.

When her health did not improve, Victor asked her what she thought of crossing alone.

“She told me: ‘Dad, I just want to be out of this place. I want to be in the United States,’ ” he said.

Lawyers working in the camp have recently become aware of the many parents choosing to send their children alone.

“These parents have been forced to consider an unthinkable choice — to save their children by sending them into the U.S. alone or to keep them in northern Mexico, where they will be exposed to severe illness, kidnapping, torture and rape,” said Rochelle Garza of the American Civil Liberties Union of Texas.

During the last week of October, Victor walked Arleth to the edge of the international bridge and watched her shuffle toward U.S. immigration agents.

“We had never been apart,” he said later, crying. “Her entire life, we had always been together. . . .

“People might hear what I did and think I’m a bad parent. But it’s the opposite. I did this for my daughter because we had no other choice to save her.”

For a week he didn’t hear from her. Then she called his mother back in El Salvador. She was at a government shelter somewhere in Texas. The details were hazy.

His mother recorded a message from daughter to father.

“Don’t worry, Dad. I’m okay,” she said. “I hope that soon you’ll be with me.”

He played the message over and over and cried.

“The truth is I don’t have much confidence that my case is going to work out,” he said. “I’m fighting it for her. But I don’t know.”