“I don’t want to be here anymore,” said a boy over a grainy video feed. “Nothing is good here.” While at the Dilley Immigration Processing Center, 9-year-old Deiver Henao Jimenez was on a video call with someone who said she wanted to help: Ms. Rachel. Wearing her signature pink headband, Ms. Rachel, whose real name is Rachel Accurso, spoke with two children being held at the center. She described the conversations as devastating. Accurso said she first became aware of the family detention center in Dilley, Texas, in January, after federal immigration agents detained the father of 5-year-old Liam Conejo Ramos in Minneapolis and sent them both to the remote, prisonlike facility. Parents and immigration lawyers have described children there losing weight after finding worms in their food, growing anxious as guards patrol and standing in line for hours for single doses of medicine. Some have suffered medical emergencies while detained. About 50 children remained at Dilley this week...