More than $200,000 has been raised in less than five days to aid the family of seven-year-old Julian Cadman who was killed in the Barcelona terror attack and whose mother remains in hospital.

A GoFundMe page for the Cadmans was set up on Monday and had raised an estimated $90,000 within its first 18 hours, according to Scott Bowman, a longstanding friend of the family and employer of the deceased boy's father.

But by Saturday morning more the site had attracted nearly $220,000 in donations from more than 3500 donors.

"It's incredible; it really is," Mr Bowman told radio station 2GB on Friday about the response. "I want to thank everybody from the $1 donation to the $5000 donation. The family is overwhelmed. It's totally unexpected."