The boy appears to grip the barrier with his fingers, leaving the impression the entire thing could be toppled with a giggle.
A French artist who goes by the moniker "JR" erected the cut-out of the boy that stands nearly 20 metres tall and is meant to prompt discussion about immigration.
On Friday, a steady stream of people drove to the remote section of wall near the Tecate border crossing, about 64km southeast of San Diego.
Border officers warned visitors to keep the dirt road clear for their patrols and not pass anything through the fence.
On the Mexico side, families scrambled down a scrubby hillside to take selfies with the artwork. Children in school uniforms played tag under the scaffolding supporting the photo.
People on each side of the wall waved to each other.
JR has done other large-scale portraits around the world, with much of his recent work focused on immigrants.
He told reporters at Wednesday's unveiling of the portrait that he was spurred by a dream in which he imagined a kid looking over the border wall.
"And when I woke up, I wondered: 'What was he thinking?'" he said. "Like for us we know all the implications, what it represents, how it divides, but for a kid, I didn't have the answer."
A year later, when JR was scouting for the perfect spot for his project, he noticed a house in Tecate near the border wall. He and a Mexican friend knocked on the door to see about the possibility of locating it around there.
After they drove away, it occurred to him that the one-year-old at the home who had been staring at them reminded him of the boy he had dreamed about.
JR and his friend immediately went back. The artist asked the woman if he could photograph her son. She knew his work and agreed.
JR told the New York Times that the idea that borders should be closed is for him, "not a discussion".
"As an artist, I try to bring back perspective," he said. "For this little kid, there are no walls and borders."
The artwork was unveiled the week US President Donald Trump said he would end a programme that has allowed young immigrants who were brought to the US without documents as children to remain in the country.
The administration also accepted more proposals for its plans to build a continuous wall along the more than 3,200km border.
JR said he did not intend for the project in Tecate to coincide with the news about the Deferred Action for Childhood Arrivals programme, known as DACA.
Sections of wall on the Mexican side have been covered with paintings of everything from butterflies to an upside-down American flag.
JR has erected other large-scale portraits in the slums of Paris, from the top of buildings in Rio de Janeiro, and set up giant photo booths from Israel and Palestine to the US.
The latest piece will remain in Tecate for a month. JR hopes people will view it from each side.