Ms Molehomi discovered the bodies of her children in Williams' one-bedroom flat.
Her estranged husband was lying beside them dazed, but uninjured, next to a noose.
Ms Molehomi said in a statement after the hearing: "I will never be able to wipe the image from my mind, nor forget the terrifying desperate moments when I scrambled to help them, to call for help from others and to do what I could for them while I waited for the emergency services to arrive.
"Try to imagine the panic, the isolation and the sense of uselessness in those moments, as I realised that my most precious children needed me the most and I was unable to save them."
The video tape found in the flat shows the children sitting in the living room with Williams initially behind the camera before he joined them on the sofa and told them: "Say, 'bye Mummy'."
The tape was labelled, "Daddy, Yolly, Theo. Byee The End".
Senior Investigating Officer Vinny Chadwick: Petros is "a cruel, self-pitying bully"
There was a note attached which said, "Play the video, made for your memories, thank you, Petros".
Williams, who had denied two counts of murder, was found guilty by the jury after 90 minutes of deliberation.
He had tried to blame his wife for the killings, but the judge said there was "no doubt" he had intended to kill them.
"It is hard to conceive any more shocking crime than a parent deliberately taking the life of his or her child," said Mr Justice Parker.
"We saw from the video that Yolanda and Theo were happy children, full of love and laughter, with a whole lifetime ahead of them and, above all, absolute trust in their father who, for entirely selfish purposes, would end their brief lives."
The couple moved to the UK in 2002 and had lived in Whalley Road since 2004. Their marriage suffered difficulties and both had affairs, the jury was told.
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