The Washington State Patrol reported the death on Sunday.
The child was identified in an accident memo as Isabella Rose Bednarski. She died Friday, troopers said.
The two-car collision occurred around 3:20 p.m. Monday on Highway 522 east of Fales Road.
It killed a Redmond couple, Stanley and Joan Kinger, 79 and 76. They were in a Ford Fusion.
The child was a passenger in a Mitsubishi driven by a Monroe man, 24. He remains at Harborview Medical Center in Seattle.
Troopers believe that the driver of the Mitsubishi was heading east and crossed over onto the westbound shoulder. The Mitsubishi hit a traffic barrel that was in some grass beyond the shoulder. The driver overcorrected before crashing head-on into the other car, troopers said.
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