Mother and son last spoke on the child’s ninth birthday, a week ago. This was no celebratory call.

Lidia Karina Souza had been released from immigration detention nearly two weeks earlier. But she could not tell Diogo, who was separated from her shortly after they reached the United States, when they would see each other again.

“Don’t cry. You are going to get a Nintendo, a birthday party. Don’t worry,” Ms. Souza, who is from Brazil, told her son. The telephone conversation was recorded and later provided to The New York Times.

They had parted ways at the southwest border on May 30. Ms. Souza was locked up. Diogo was flown to Chicago, where he was placed in a shelter. Ms. Souza was released on June 9 and allowed to join relatives in Hyannis, Mass., but it is still not clear when her son will rejoin her.

“I am going to do everything to get you out of there,” she told him on the call. “It’s so many papers they need.”