Julian's mother, Jom, has since been found in hospital in a serious but stable condition.
Julian Cadman became separated from his seriously injured mother Jom during the confusion on Las Ramblas on Friday.
"By about 8.30am we found out that it was her in the hospital".
"Julian is 7 years old and was out with Jom and they were separated".
"Please share, especially if you have family or friends in Barcelona".
"We condemn in the strongest possible terms these brutal and cowardly attacks, and in the case of Las Ramblas, clearly created to harm and affect tourists who were visiting the area", Bishop said.More news: Government fails to censure Penny Wong over Barnaby Joyce citizenship issue
The Prime Minister said United Kingdom authorities were "urgently looking into reports" that the child, who has dual British nationality, is missing.
Speaking after Julian's grandfather made a plea for information, Theresa May said United Kingdom authorities were "urgently looking into reports" that a child, who has dual British nationality, is missing.
Police said they also shot dead on Thursday a man who had driven a auto into a police checkpoint in Barcelona, though they had no evidence he was connected with the van attack.
The Foreign Office has confirmed that a "small number" of Britons sustained injuries in the attacks in Spain.
The dead, the missing and the injured from the deadly terrorist attack Thursday came from dozens of countries in Europe, Asia and the Americas to spend time in Las Ramblas, one of the most joyous, vibrant streets in Europe, only to be mowed down by a white van.More news: Driver in fatal Charlottesville wreck identified as James Alex Fields
Italy's La Repubblica newspaper reported that he had been walking with his wife, who managed to pull their five-year-old son from Bruno's hand to safety at the last moment.
They are working on the theory that the attacks in Cambrils and Barcelona are linked to a gas explosion at a house in the town of Alcanar on Wednesday that killed one person.
Some 130 people from 34 different nationalities were injured in the attacks, emergency services have revealed.
Police said they had arrested a Moroccan and a man from Spain´s north African enclave of Melilla, though neither was the van driver.
The mother's uncle also died in the attack, authorities said.More news: German growth slower, but still strong ahead of election