Police say the family was jaywalking while trying to cross Pacific Highway, just one block south of Des Moines Road.
The family was standing on the median strip in the road, waiting to cross. Fire officials say two cars stopped to let the family cross, but a truck kept driving.
"They started to cross," Kyle Ohashi with Kent Fire Department said, "a car in the far right lane did not see them, continued on and struck the family of four killing one female."
Dwayne Curtis was walking through the Dollar Tree parking lot when the family was hit, and saw the gruesome scene.
"She was pinned against that pole over there," Curtis said. "They backed the truck up, and somebody pulled the baby from underneath the truck."
Police are reconstructing the scene to figure out exactly what happened. They say the driver of the truck is cooperating with the investigation.