A Chicago-area woman has been charged with killing her 5-year-old son and a 7-year-old girl as she babysat them, allegedly stabbing each dozens of times in a bedroom this week as they pleaded for their lives, authorities say.

Elzbieta Plackowska, 40, of Naperville, Illinois, allegedly told police she instructed her son Justin Plackowska and the girl, Olivia Dworakowski, to kneel and pray before she fatally stabbed them and two dogs at the girl's home on Tuesday evening, Naperville police said.

Plackowska -- charged Thursday with two counts of first-degree murder -- allegedly gave investigators a number of reasons why she killed the children, the most recent of which was that she was upset with her husband and wanted to cause him pain, according to police.

"She began stabbing her son Justin and told him he was going to heaven tonight," DuPage County prosecutor Bob Berlin told reporters Thursday, citing Plackowska's alleged statements to police. "He pleaded for his life and told her to stop, but she continued stabbing him until he was dead."

Plackowska was being held Thursday without opportunity for bail. If convicted, she would face a mandatory sentence of life in prison with no possibility of parole.

CNN's attempts to reach lawyer Mike Mara, a DuPage County public defender representing Plackowska, weren't immediately successful.

Police officers found the children's bodies in Olivia's home Tuesday night after Elzbieta Plackowska, covered in blood, arrived at a nearby friend's house and claimed she had been robbed, police said.

The friend called police. Around the same time, Olivia's mother called police to say she had come home from work and couldn't enter the house because there was a strange lock on the door, and that she couldn't find Plackowska or the children, Berlin said.

Officers forced entry into the home and "walked into a scene of unimaginable horror," finding the bodies on the floor of the master bedroom, Berlin said.

Investigators said Plackowska offered several explanations, including that she believed an intruder killed the children, police said. Later, she told police that the children had evil inside them and she needed to drive the devil out, authorities said.