Amber Kyzer told a court in South Carolina that Tim Jones Jr "did not show my children mercy by any means, but my kids loved him".
The 37-year-old was convicted in May of killing the siblings, aged one to eight, at his home near Lexington on 28 August 2014.
The jury is considering whether Jones should be executed or jailed for life.
"I hear what my kids went through and what they endured," Ms Kyzer said in court on Tuesday.
"And as a mother, if I could personally rip his face off I would.
"That's the mom in me. That's the mama bear."