Writer-producer Ben Wexler said, "This man, Corey Lewandowski, shouldn't be able to leave his house without being punched in the face".
"Did you say "womp womp" to a 10-year-old with Down syndrome?"
"How dare you? How absolutely dare you, sir?" asked the visibly upset panellist.
Lewandowski had said prior to Petkanas relaying the story of the girl with Down syndrome that "as soon as you cross the border illegally, you have committed an act which is a disservice to all of the people who have stood in line and waited to get into this country legally".
The Department of Justice moved earlier this year to prosecute illegal border crossers to the fullest extent of the law, resulting in children being separated from their families and detained as the adults are prosecuted.
He said the girl and her brother went to a facility in McAllen, Texas, while her mother was sent to a different centre, an hour away.
During the panel, Petkanas mentioned the story of the Mexican girl, who was reportedly sent to a Texas shelter despite her father being a U.S. resident who lived about 60 miles away, according to The Washington Post. Their father, a legal USA resident, is working with the United States consulate to get the girl released.
"There is no low to which this coward Corey Lewandowski won't sink", former Fox News star Megyn Kelly tweeted.