“Your loved ones have not died in vain,” he told the group of so-called Angel Families at the White House on Friday, BBC reported.
Trump has faced global condemnation for the US immigration policy that has seen more than 2,000 migrant children stripped from their families.
He bowed to public pressure and reversed the policy earlier this week.
The president signed an executive order on Wednesday to end the separate detention of undocumented immigrant children from their parents after illegally entering the country.
As part of the the administration’s “zero-tolerance” policy, which seeks to criminally charge and jail undocumented border crossers, children cannot legally be held with their parents. They have instead been kept in separate facilities.
“These are the American citizens permanently separated from their loved ones,” he said on Friday, before introducing family members of victims.
“I cannot imagine it being any worse, but we promise to act with strength and resolve.
“We’ll not rest until our border is secure, our citizens are safe and we end this immigration crisis once and for all,” Trump added.
Laura Wilkerson, whose son was killed in 2010 by an undocumented immigrant, told audience members: “None of our kids had a minute to say goodbye. We weren’t lucky enough to be separated for five days or 10 days.
“We were separated permanently.”
Approximately 2,300 children have been removed from their families since Mr Trump’s “zero-tolerance” policy began in May, and housed in detention centres run by the Department of Health and Human Services.
Some shelters, including three in Texas, house so-called “tender age” children, who are under five years old.
About 500 children have been reunited with their families since May, a Homeland Security official said on Thursday. But it is unclear how the remaining families affected will be reunited.
Images of children in metal cages, and audio of young migrants crying out for their parents sparked protests and drew international condemnation from world leaders and organisations.