"Oh, it's happening," Trump told the child on the South Lawn of the White House after the child apparently urged the president to "keep building" it. "It's being built now."
Trump made the comments after stopping by to talk with children who were coloring cards for service members.
"Can you believe that? He's going to be a conservative someday!" Trump said.
Trump has vowed since running for president to build a border wall to deter illegal immigration. He has struggled to secure funding from Congress to build new walls, though existing fencing on the border has been replaced during his presidency.
Earlier this year, Trump declared a national emergency to free up billions to use for the wall, though the plan has been targeted in lawsuits by critics.