File photo: Migrant children housed at a detention centre near the Mexican border in Tornillo, Texas are shown walking in single file.
All migrant children under age 5 will be reunited with their families by Thursday, Reuters quoted a Trump administration official as saying.
The official said all children who are "eligible" will be reunited with their families.
The US has already missed one deadline to reunite the children with their families. A judge had earlier ordered that all children under 5 be reunited with their families by Tuesday.
The children are in custody because of Trump's "zero-tolerance" immigration policy under which migrant children were separated from their families if caught crossing illegally into the US.
Trump had finally backed down from his policy after worldwide outrage.