Brittney Sue Humphrey, of Lebec, pleaded no contest to the kidnappings in July. She faces up to 26 years and four months in state prison, the Los Angeles County District Attorney's Office said in a statement.
Sentencing for Humphrey was postponed to Sept. 20.
According to prosecutors, Humphrey and her boyfriend, Joshua Aaron Robertson, abducted Kimberly Harvill's children - Joslynn Watkins, Brayden Watkins and Rylee Watkins in August 2016.
The young victims, each under the age of 6 at the time, were later found abandoned in a New Mexico motel.
A motorist discovered Harvill's body along a road near Gorman during a rest stop on Aug. 14, 2016. Investigators said she had been shot to death.
Robertson and another man are awaiting trial for the mother's murder.