Most children who are experiencing problems so far display acute anxiety around routines that separate them briefly from their parents, such as when the adult bathes or goes into another room, said those who are monitoring these reports.

“These kids don’t want to be without their mothers; it triggers a feeling of abandonment, or that their mother will be taken away from them,” said Luana Biagini, a paralegal who has been working with reunited Brazilian families.

“I have mothers complaining that their child was more outgoing and talkative, and now they are quiet and unresponsive. Some take a while to process information or a situation, and Mom has to say, ‘Hey, hey wake up,’’’ said Ms. Biagini, who works at the Jeff Goldman law firm in Boston.

The recent round of separations was hard on children in part because the parents themselves were so traumatized, according to those who have worked with the families. In some cases, children were torn from their parents’ arms amid tears and pleas. Other children appear to have been duped — told they were being taken to play with other children but never returned to the parent.

Often, parent and child were prevented from communicating for weeks or longer. In limbo and confused, many children likely internalized the separation as a punishment, experts say.

Decades of research have concluded that children traumatically separated from their parents have a high likelihood of developing emotional problems, cognitive delays and long-term trauma. More recent studies have found that separation can impair memory and normal production of cortisol, a hormone produced in response to stress.