Julian has not been seen since the terror attack on Las Ramblas on Thursday night, while his mother remains in a serious condition in hospital.

His father, Andrew Cadman, arrived in Barcelona late Saturday afternoon after making the long flight from Sydney.

He was met by Australian consular staff at Barcelona airport and driven straight to the Ciutat de la Justicia. According to the Catalan government, the Ciutat de la Justicia is where victims of the Las Ramblas attack are being identified by forensic experts.

Mr Cadman appeared distressed and was accompanied by an unnamed woman.

A few hours earlier, Spanish police had said: "Neither we were searching nor we have found any lost child in the Barcelona attack. All the victims and injured have been located."

Mr Cadman stayed at the the Ciutat de la Justicia for more than an hour, and was then taken to the Barcelona hospital where his wife Jumarie 'Jom' Cadman is being treated for serious injuries.

A Vall d'Hebron hospital spokesman said he had been asked by the family not to talk to the media about the family's situation, beyond confirming the woman remained in a "menos grave" medical condition, which translates as "less serious".

Fairfax understands Mr Cadman may choose to spend the night there, and the hospital will also offer him trauma counselling.

A witness has told how Julian's seriously injured mother begged for information about her son as she lay wounded following the attack, News Ltd reported.

"I was at her side helping her, telling her, 'be calm, don't worry'," Fouad Bakkali told the newspaper group.

"She was asking all the time about her little boy. She asked me, 'where is my son?'.

"I told her, 'he is good, it will be OK'."

He said it appeared she had suffered broken legs, a back injury and a head injury.

Prime Minister Malcolm Turnbull has asked Australians to pray for Julian.

Mr Turnbull said the "shocking terrorist attack" had resulted in Australians being injured and grave fears were held for the little boy.

The mother and son were visiting Barcelona for a wedding this weekend when they were caught up in the deadly terror attack.

Mr Cadman stayed behind in Australia.

The boy's aunt, Claire O'Sullivan, and his grandfather, Tony Cadman, from Sydney, posted pleas on Facebook.

His distraught aunt Maricar Querimit Estera said his "family were waiting for him".

"My nephew is still missing ... they need help to find him quickly ... pls share," she posted on Facebook.

Another family friend, Sarah Auzner, pleaded with the public to share his photo.

"Please, please share his picture so he can be with his family," Ms Auzner wrote.

"We are beside ourselves with worry. Andrew and Jom Cadman we are praying for you."

Foreign Affairs Minister Julie Bishop confirmed that four Australians had been hurt in the attack, while one person is missing.