Plackowska told investigators that the children had been jumping on the bed. She ordered them to kneel on the floor and pray and then stabbed them both dozens of times as they begged for their lives, Berlin said.
Covered in blood, Plackowska drove to a Catholic church. Berlin said Friday that it was the same one where she went to confession before the slayings.
Finding the building closed, she called the church and left a message saying she had "done something bad" and needed help, Berlin said. She then went to a friend's home where her adult son was staying. Police arrested her there.
Berlin said Plackowska had been babysitting for Dworakowski since the beginning of the school year, although Justin and Olivia went to different schools and it wasn't clear how or when the two women met. Plackowska babysat Olivia numerous times as her mother works various shifts as a nurse, he said.
Don Babwin reported from Chicago. AP News Researcher Jennifer Farrar contributed to this report from New York.