Lianne Smith (45) told a court she had used plastic shopping bag to suffocate 5-year-old Rebecca and 11-month-old Daniel to death in May 2010.
The Girona provincial court in north-east Spain was stunned to hear Smith recounting her macabre killings in a recorded interview with the police.
She said as Daniel struggled for breath, she drew the bag closed around his neck and 'did not stop squeezing' until he was dead, The Daily Mail reported.
Then she did the same to her daughter before giving her children a cuddle and spending the night beside their bodies.
Smith claimed she killed her children because she feared they would be taken from her by social services after her partner