PROSECUTORS say a suburban Chicago woman stabbed her 7-year-old son more than 100 times and a 5-year-old girl about 50 times and slit both their throats because she was angry with her husband.

Elzbieta Plackowska, 40, told investigators she stabbed the children and slashed their throats on Tuesday night at the home in Naperville because she was angry with her husband, a truck driver who was often away, DuPage County State's Attorney Robert Berlin said.

Ms Plackowska told investigators her husband left her alone to care for her son Justin and work as a maid, which she felt was beneath her, Mr Berlin said at a bond hearing.

"She told the detectives that she thought by killing Justin she would make her husband hurt the way she hurt in their relationship," Mr Berlin said.

Ms Plackowska told investigators she found Justin and Olivia Dworakowski - whom she was babysitting that night - jumping on the bed, Mr Berlin said.

"She had grabbed the knife and she made both victims kneel and begin praying. She began stabbing her son Justin and told him he was going to heaven tonight. He pleaded for his life and told her to stop. But she continued stabbing him until he was dead," he said at a news conference after the hearing.

"She then turned her attention to 5-year-old Olivia and began stabbing her with the knife multiple times. She, as well, pleaded for her life, but the defendant continued to stab her until she was deceased."

Ms Plackowska killed Olivia because she had witnessed the attack on Justin, Mr Berlin said.

The slayings took place at the home of Olivia's mother who works nights as a nurse and had left her daughter in the woman's care before. Ms Plackowska also stabbed the two family dogs.

"In all my years in law enforcement, this is the most gruesome and horrific crime scene that I have seen," Naperville Police Chief Bob Marshall said at the news conference. Naperville is 40 kilometres west of Chicago.

Investigators found two blood-stained knives; one in the kitchen sink and another in Ms Plackowska's car, Mr Berlin said.

After the killings, Ms Plackowska left a message on the answering machine of a Naperville church saying she had done something bad and needed assistance, Mr Berlin said.

Ms Plackowska initially told investigators that a stalker had broken into the home where Olivia lived with her mother, and that it was the stalker who killed the children. She later said the children were evil and that she was fighting the devil in the house, Mr Berlin said at the hearing.

When police confronted her about the inconsistencies during hours of questioning, Ms Plackowska admitted that she lashed out because she was angry with her husband, he said.

Mr Marshall said several of the investigators knew Olivia after she was reported missing in August when she went to the wrong kindergarten class on the first day of school.

The officers told him "she was such a sweet and loving little girl," Mr Marshall said.

Ms Plackowska arrived in the United States from her native Poland on a tourist visa 12 years ago, said Mr Berlin. She is not a US citizen and authorities were trying to verify her immigration status. Mr Berlin said she has no prior history of violence, although she had a misdemeanor DUI from about 10 years ago.

DuPage County Judge John Kinsella ordered Ms Plackowska held without bond. Ms Plackowska didn't speak during the hearing other than to indicate she could not afford an attorney. The judge appointed a public defender.