Lianne Smith, 45, a former child protection worker, faces up to 34 years in prison after she was found unanimously guilty by a jury of nine at the Provincial Court in Girona on Thursday.
She had admitted to suffocating Rebecca, five and 11-month-old Daniel on May 17, 2010 in the Miramar Hotel, Lloret de Mar, after her long-term partner Martin Smith was arrested on child sex offences.
During the trial, the jury were shown a video of Lianne Smith confessing the crime to police as she sat in the room at the Miramar Hotel where the murders took place.
"I know it's not right taking a life but I felt that I was in a corner. My intention was that my children and me together would go," she said in the video.
She added that she had given the children the "perfect" three-day holiday before she killed them.
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"This was the end of the road, I knew my children would be taken back to England," she said.
Smith will serve her sentence in a regular prison somewhere in the province of Catalonia, not in a psychiatric hospital.
She was expected to be put on suicide watch and will be sentenced at a later date.