Washington State Patrol troopers said the causing driver had smoked marijuana earlier in the day and she has since been arrested.
Five vehicles were involved in the pileup, which happened at about 6:20 p.m. on Highway 2 in between Sultan and Monroe.
Troopers say one of the vehicles, a Chrysler minivan, was stopped to make a left turn and a Chevy Blazer behind it had slowed down and was coming to a stop.
Investigators say that's when Heather Lee, driving a Dodge Durango, came up from behind the Blazer and slammed into it, triggering a chain reaction as the Blazer was shoved into the minivan from the force of the impact.
The Dodge Durango then skidded and veered left into the oncoming traffic, crashing head-on into a Dodge Caravan. Another vehicle traveling behind the Caravan then steered right to avoid a collision and wound up in the ditch.
Five children riding as passengers in the Blazer were injured, along with the driver, a 23-year-old Monroe woman. The injured children were identified as a 13-year-old boy, a 14-year-old boy, an 11-year-old girl, a 7-year-old girl and a 10-year-old boy.
A 1-year-old boy and a 5-year-old girl riding as passengers in Lee's vehicle were not hurt.
The driver of the Dodge Caravan, a 37-year-old Startup woman, also was injured.
All of the injured were taken to local hospitals. The more critically injured were airlifted to Harborview Medical Center in Seattle, including the 10-year-old boy who had a crushed skull, investigators said.
"First thing when I hear kids are involved, immediately your heart goes out to the kids," said Trooper Keith Leary.
Lee was treated and then booked into Snohomish County jail for investigation of three counts of vehicular assault.
According to the statement for probable cause, Lee told investigators after the crash that she had turned her head to the back of the car because her two kids were fighting and screaming in the back seat and didn't notice traffic had slowed ahead of her until it was too late. She also admitted to detectives she had smoked "a bowl" of marijuana earlier in the day.
Valerie Ballard, who was driving the Chrysler minivan, was not injured and is hoping everyone else involved recovers.
"I feel so fortunate that God spared me," she said. "I hope no one dies. It would be a tremendous and wonderful thing."
Ballard has lived along the highway her entire life and said she's seen too many car crashes. She said the state should widen the road to four lanes to help avoid similar accidents.
"It needs to be fixed and it needs to be fixed now," she said. "Not yesterday, not 20 or 30 years from now, it needs to be fixed now."
The road was closed for hours as troopers cleared the scene.