Shanda Yenglin lowered her thermostat to 53 degrees Sunday night, drugged the children with milkshakes and put them to sleep in a minivan spewing carbon monoxide into a garage, Waterford Township Police Chief Daniel McCaw said.
The children survived because one girl woke up, found her mother dead on the garage floor and got help, the chief said.
Yenglin, who left a suicide note in the van, was dead at the scene. Two boys, 10 and 11, were found unconscious, and the girls, 13 and 14, were wandering in the house confused.
McCaw said Tuesday both girls have been released from the hospital, and the boys "were doing well."
The Detroit News reports that Oakland County court records show Yenglin lost custody of the children last May after a state abuse investigation. She was later granted supervised visits and then unsupervised visits in January.
A friend, Orlando Blanco, told the Detroit Free Press Yenglin was "a very caring mother who did everything she could for those kids."
"Maybe she thought the children would see her in the afterlife," he said.