Brett and Naghmeh King could see the five-year-old at the Materno-Infantil hospital in Malaga, but were not allowed by the medical authorities to remove him.
They had gone to the country, where they have a holiday home, after taking Ashya out of Southampton General Hospital, where he was being treated, without the permission of doctors last Thursday.
The couple who had been released from prison in Madrid on Tuesday after the Crown Prosecution Service dropped an extradition bid.
A court in England is due to make a final decision on the right course of treatment after hearing representations from Southampton NHS Foundation Trust and the family on Monday.
However, that decision could be speeded up if agreement is reached between the parties.
The parents hope to take Ashya to Prague for proton beam therapy after more than £121,000 was raised from donations and by a children's cancer charity to enable the procedure to go ahead.
Before seeing her son, Mrs King said she had been "crying and crying" as she described the torment of being unable to help her son from the jail cell where the couple haad spent a total of four days.
Mr King said his heart had been "aching" to see Ashya and said they had been treated like terrorists
Southampton General Hospital's chief paediatrician Dr Peter Wilson said: "At no stage did they say they were going to take Ashya and go to Prague."