Mobile Detective Angela Prine testified Monday that Heather Leavell-Keaton admitted the poisonings to another inmate while confined at the Mobile County jail.
She told the inmate that she and common-law husband John DeBlase wanted the children dead because "it would be better if they had less responsibilities," the detective said
At a previous hearing, investigators had said Leavell-Keaton told them DeBlase fed rat poison to the children.
Prine's testimony came in a preliminary hearing on capital murder charges against the 22-year-old stepmother.
Leavell-Keaton's attorney, Richard Horne, said she is functionally blind and not capable of some the things she's accused of doing. But Circuit Judge Rick Stout found probable cause to send the case to the grand jury.
The two children were reported missing in November. The body of 3-year-old Chase DeBlase was found near Vancleave, Miss., on Dec. 8 and the body of 4-year-old Natalie DeBlase was found near Citronelle three days later. Their father is jailed on murder charges.
Prine testified that jail inmate Roseanna Russell was assigned to help Leavell-Keaton because of her blindness and the defendant began talking to her about the case. The detective said Russell learned the couple first poisoned their dog with antifreeze "to find out how long it would take to kill a living thing." Then they started giving antifreeze to the children, who got progressively sicker.
Natalie died March 4, 2010, and they dumped her body, the detective said. They stopped poisoning Chase for a few months but started back because he began crying and asking for his sister in front of others, she testified.
Chase died June 20. On the way to dump his body in Mississippi, they stopped at a video game store to buy two games because they no longer had the responsibilities of the children, the detective said.
If convicted of capital murder, Leavell-Keaton could be executed or sentenced to life in prison without parole.