Yazmin Juarez, an asylum seeker from Guatemala, spoke out at a congressional hearing staged amid a series of scandals over poor conditions suffered by detained migrants that has rocked Washington.
She said 19-month-old daughter had contracted a deadly lung infection during a 20-day detention near the US border with Mexico.
The migrant mother's story left some members of a US House panel, including New York Democrat Alexandria Ocasio-Cortez, visibly shaken. Ms Ocasio-Cortez and several others hugged the woman after she spoke.
Yazmin Juarez said she fled to the United States last year with her 19-month-old Mariee "because I feared for our lives in Guatemala."
She crossed the border and claimed asylum but says she and Mariee were "locked in a freezing cold cage for a few days," then moved to an ICE detention center, when her daughter became ill.
"I begged the doctors and medical staff to give her the care I knew she needed but they didn't," the mother said. "When ICE finally released us, I took Mariee... to a doctor and then to the emergency room. But it was too late. Mariee never left the hospital."
"The world should know," she said. "It can't be so hard for a country like the United States to protect kids who are locked up." The mother testified at the House Committee on Oversight and Reform that was examining treatment of refugees in US.