The reunification videos should be heartwarming. Yet footage of migrant parents meeting their little ones for the first time following months of separation often shows a much more disturbing encounter.

In one, Guatemalan asylum-seeker Hermelindo Che Coc meets his 6-year-old at the Los Angeles International Airport, carrying a birthday balloon and a present. When he sees his son, the boy stares back vacantly. He offers no smiles or cries of joy. Instead, his body is rigid when his father embraces him.

The New York Times reported similar stories of parents' worst nightmares — their stunned children no longer recognizing them and calling out instead for the social workers at the shelters where they'd been held after being taken from their parents at the border.

Watch video 02:37 Share Kids still wait for reunion Send Facebook google+ Whatsapp Tumblr linkedin stumble Digg reddit Newsvine Permalink https://p.dw.com/p/32BWk Hundreds of migrant children still not with parents

"The main thing to realize is that the children are very traumatized by the separation and the conditions in the detention center," Dr. Elizabeth Barnert, an assistant professor of pediatrics at UCLA, told DW. "Then the reunion is a very confusing and intense time. The children feel happiness but also a lot of confusion and anger."

Barnert's research focuses on the separation and reunification of children who were disappeared during El Salvador's civil war. She warns that children may blame their parents for abandoning them.

"In a child's psychology the natural tendency is to think, 'Mom, you failed to protect me.'"

Concern over efforts to curb illegal immigration grew in the US as images of immigrants in cages hit the news

As the Trump administration reaches the court-ordered July 26 deadline of reuniting all children over the age of five with their parents, the difficult reunifications that already have taken place highlight how much damage splitting thousands of families at the border has inflicted on families. Now doctors warn the children who were pried from their parents are likely to face physical, mental and emotional long-term damage.

Dr. Jack P. Shonkoff, director of Harvard University's Center on the Developing Child, released a statement warning that depriving a child of support from his or her primary caregiver can have serious impacts not only on learning and behavior, but also on long-term physical health.

"Above and beyond the visible distress 'on the outside,' this overwhelming experience triggers a massive biological stress response inside the child, which remains activated until that familiar caregiver returns." Shonkoff wrote: "[E]ach day we fail to return these children to their parents, we compound the harm and increase its lifelong consequences."

Toxic stress raises risk of disease, diabetes, cancer

Dr. Lanre Falusi, a pediatrician at the Children's National Health System in Washington, DC, treats mostly immigrant children. She has patients who came to the United States after experiencing abuse and the threat of kidnapping. One of her patients — a 10-year-old boy — was threatened with death if he didn't join a gang.

She says the extreme stress of separating a child from a parent exacerbates the existing trauma these children have faced. When that results in toxic stress response, the consequences can be devastating.

"We see changes in their stress hormones; we see changes in their brain development. In the long term that also can manifest as mental health illnesses like PTSD [post-traumatic stress disorder], anxiety and depression. Kids who have experienced toxic stress have higher risk of suicide as they get older," Falusi told DW.

She said younger children are particularly vulnerable. Toxic stress can even lead to physical health problems.

"The changes in their hormones actually cause physical changes in their bodies. So increased risk of heart disease, diabetes, liver problems and increased risk of cancer."

Watch video 02:08 Share Migrant children in US in limbo Send Facebook google+ Whatsapp Tumblr linkedin stumble Digg reddit Newsvine Permalink https://p.dw.com/p/30HdU Fate of migrant children in US detention still uncertain

The unexpected health risks of escaping to the US

Supporters of tough US immigration laws believe a credible threat of harsh treatment is the only way to deter prospective immigrants from crossing the border. Yet Falusi believes her patients fleeing violence don't see any other option.

"When these parents take their kids and their belongings in the middle of the night and they flee, they're not thinking they have a choice," Falusi told DW.

For now the focus is still on the daunting logistics of bringing the separated children back together with their parents as quickly as possible. Three days before a reunification deadline imposed by a federal judge in California, the US government said that only 879 parents had been reunited with their parents, leaving some 1,700 others in the 5-17 age group still waiting. In the same filing, the government said that 463 are no longer in the country even though their children are still in the US. Officials previously had admitted that some of those parents had been deported.

Children walk in a single file to tents at a detention facility near the Texas-Mexico border

Although the Trump administration changed course after widespread public outrage over separations at the border, the policy reversal does nothing to address the damage caused already or provide resources for fixing it.

Barnert says it's essential to consider the next steps for the children after they've been returned to their parents. She says it's important that they are evaluated by mental health professionals and offered the appropriate treatment.

"Once the families have reunited, in addition to needing psychological support, these kids will probably all have depression, anxiety, and some component of PTSD," Barnert said.

For the affected families who thought they had escaped the worst for a better life, the United States has instead offered them more trauma.