By James Blears
The seven-year old Guatemalan girl was accompanied across the border by her father.
They were detained by the US Border Patrol trying to cross from Mexico to the United States.
Officials say the child, who hadn't eaten or drank liquids in several days, had developed a fever and then her condition worsened with seizures.
She was immediately flown to a hospital in El Paso, but staff there were not able to save her life.
A distressed US Border Patrol spokesman said: "Despite the best efforts of our medical team, we were unable to stop this tragedy."
There are hundreds of children, many of them very young, traveling with their parents all the way from Central America, desperately trying to escape dire poverty, but also organized crime, which tries to recruit children into their ranks at a very early age.
Mexico, the United States, Guatemala, Honduras, and El Salvador are working together to try and establish a Central American Development Plan, which will offer jobs and discourage migration.