"We have found Jom (my daughter-in-law) and she is serious but stable condition in hospital", the boy's grandfather, Tony Cadman, wrote on Facebook.
Hopes were fading for seven-year-old British boy Julian Cadman on Saturday as Spanish authorities revealed they were no longer searching for any missing youngsters.
The boy's concerned aunt Maricar Querimit Estera, posted onto Facebook saying "pray for my nephew who [is] still missing in Barcelona attack. your family [is] waiting for you".
Mr Cadman, from Sydney, arrived at the hospital after visiting the justice centre.
He said: "My Grandson, Julian Alessandro Cadman is missing".
It is thought the youngster got lost after he was separated from his mum who is understood to have been badly injured during the attack.
His wife, Martina, who was carrying their one-year-old daughter in a sling-style baby-carrier, told friends that she had pulled their son to safety at the last moment, according to Italian media reports. "All the victims and injured have been located".More news: French police arrest man carrying pistol at train station
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Police have made three arrests in connection with the incident, including Driss Oukabir, a 28-year-old Moroccan who is believed to have rented the van.
The terrorists behind deadly attacks in Spain that killed 14 people were preparing bigger atrocities than those they carried out, police said.
Meanwhile, Germany's foreign office says at least 13 German nationals were injured in the attack - "some of them seriously, so seriously that they are still fighting for their lives", says foreign ministry spokesman Martin Schaefer.
Foreign Affairs Undersecretary for Migrant Workers Affairs Sarah Arriola said the woman was with her son in Barcelona to attend the wedding of a cousin from the Philippines.
She said the missing boy had dual nationality; however she did not name him. He was holding the hand of his five-year-old son but was reportedly died when he tried to shield his family from the vehicle.
"Sadly, I must tell you that we do believe that a number of British nationals were caught up in the attack", she said.
The Civil Safety Agency of Catalan said an eighth victim - a Portuguese citizen - had now been identified.