"We're going to have strong, very strong borders, but we're going to keep the families together", said Trump, who added that he didn't like the "sight" or "feeling" of children separated from their parents.
Given those figures, the centers would be full within eight days.
"We are pleased that the president is calling a halt to his inhumane and heartless policy of separating parents from their children", said Peter Schey, the lawyer in a lawsuit that resulted in a key agreement governing the treatment of migrant children in detention called the Flores settlement.
Lost amid the furor over the Trump immigration policy that separated families and imprisoned asylum seekers is a much younger and smaller group of 30 to 60 migrant teenagers already caught up and often locked up in Oregon.
Democratic Sen. Dianne Feinstein, of California, shared similar sentiments, tweeting: "It's extremely troubling that the president's executive order would require immigrant families with children to be detained indefinitely". Family detention is also inhumane and harmful to children.
In the Oval Office, Trump said he had also heard from his daughter and aide, Ivanka Trump, about the policy.
"I'll be signing something in a little while that's going to do that", Trump told reporters.
Until Wednesday, Trump had held firm on the matter.
"Huddling with the GOP at the Capitol on Tuesday evening, Trump stopped short of giving a full-throated endorsement to immigration legislation meant to unite the moderate and conservative wings of the House Republican conference". Once the party of compassionate conservatism under President George W. Bush and a party that supported a massive immigration bill under President Ronald Reagan, members admit immigration has grown more and more contentious and less and less a topic they want to debate openly on the floor.More news: Black Panther and Stranger Things dominate
Even if the president succeeds in stopping those heart-rending scenes of agonized children, the larger crisis won't end until leaders on both sides start taking political risks to find a compromise solution.
On May 7, Sessions discussed the "zero-tolerance" policy for illegal entry on the southwest border.
Still, he said the president's dramatic reversal on separating children won't solve anything: "I see that as placating people".
Since 5 May, 2,300 children have been separated from their families at the border in Mexico.
At a meeting with lawmakers and officials, Mr Trump said: "The Republicans want security and insist on security for our country".
Until Wednesday, Trump claimed that Congress had the sole authority to end the child separation policy - by passing new legislation. The order will nearly certainly prompt legal challenges.
In his 1995 State of the Union address, President Bill Clinton said: "All Americans, not only in the states most heavily affected, but in every place in this country are rightly disturbed by the large numbers of illegal aliens entering our country".
What exactly the order will say is still continued to be worked out, with ongoing conversations between the White House, the Justice Department and Homeland Security, the source explained.
By Wednesday morning, he had changed course. The White House is hoping to get around a 1997 settlement that prohibits the federal government from keeping children in immigration detention for more than 20 days.
Activists are especially concerned that there isn't a formal system for reuniting immigrant parents with their children after their court proceedings are over.