The parents of five-year-old Ashya King are planning legal action against Southampton General Hospital.
Brett and Naghmeh King's lawyer said the "desperate" couple are considering "demands" against the hospital which they removed their son from last Thursday.
The couple must be held for up to 72 hours while a court in Madrid considers whether to grant a British extradition request.
Juan Isidro Fernandez Diaz, the couple's lawyer in Spain, said they are "so sad", adding: "They are going to prepare legal demands against the hospital in Southampton. Legal action will be against the hospital."
Mr Diaz told BBC's Radio 5 Live that the couple "wanted the very best for their child".
Ashya King's parents told the Daily Mail they fled the UK after being told NHS treatments would leave him with life-long disabilities.
The head of the police force which applied for a European Arrest warrant against the parents of Ashya King has now urged their release.
A British mother whose son's life was saved by proton therapy has told ITV that she defied NHS doctors' advice to pursue the treatment.