The Popstar to Operastar contestant performed Nella Fantasia earlier this month and said it was for “someone very special”.
Later she confirmed the special person was Archie, her son with fiancé Jamie Lomas.
She told New: “It was an emotional song and a lovely tribute. The line, 'I dream of souls that are always free' was the hardest to sing.
But knowing I had so much support got me through."
Her former Hear’Say bandmate Myleen Klass hosts the singing show.
Kym was voted off at the semi-final stage last week along with Shakespears Sister singer Marcella Detroit.
Their departure leaves Bernadette Nolan and Darius Campbell to compete for the Popstar to Operastar title on next week's grand final.