IN a shocking jailhouse con­fession, evil child killer Susan Smith revealed never-heard-before details about what she says REALLY happened on the day she heartlessly drowned her two young sons in her car!

The heinous 1994 crime riveted the nation, and now, in a block­buster ENQUIRER interview, a former prison cellmate of Susan’s claims the 42-year-old mother from hell told her how she intend­ed to join her boys – Michael, 3, and 14-month-old Alex – in their watery grave.

But the psycho killer insisted her twisted suicide plan went awry when the car got away from her, says Christie Leann Smith, who served time with Susan at South Carolina’s Leath Correctional Insti­tution.

According to Christie, Susan blurted out: “I should have died with my babies! I was sup­posed to be in that car. But I got out to push it, and I couldn’t stop the car from rolling. It was moving too fast, and I couldn’t catch up to it. It went into the water, and they were gone!”

Christie doesn’t believe a word of it.

“The whole time Susan was talking, I was thinking, ‘She’s full of crap,’ ” Christie told The ENQUIRER. “I knew she had no intention of kill­ing herself and was just spinning some lame story in a pathetic at­tempt to make herself look a little better.”

Indeed, in her written confession, Susan claimed she deliberately watched as the car rolled into a lake near her Union, S.C., home with her boys strapped into their car seats. And the cold-hearted vixen murdered her innocent chil­dren simply because they had become a stumbling block to her affair with a wealthy local man.

She’s now serv­ing a life sentence for their murders.

While in prison, Susan has had several lesbian lovers, in­cluding a drug-addicted shoplifter, who died of an overdose after her release, and a black cocaine dealer named Pamela “Big Daddy” Morgan, who was released in 2008.

“Susan always plays the feminine role,” said Christie, who was incar­cerated for fraud and writing bad checks. “She likes big, tough convicts who can protect her from other inmates and make love like a man.”

After ballooning up to a hefty 180 pounds, Susan has slimmed down to 135 since starting lesbian romps with her new lover, who’s serving 15 years for armed bank robbery, said Christie.

Christie adds that the monstrous mom paid her to watch for guards while making love with her girlfriend in their cells, closets and even the prison cafeteria freezer.

“Susan gets money from men she meets through prison pen pal web­sites, and she’d put $30 to $50 a week in my prison commissary ac­count,” said Christie.

“She really has it made in prison. She’s popular with other inmates, teaches GED classes and has a full-time lover.

“She certainly has it a lot bet­ter than her boys, who never really lived.”