Investigators say the driver of the sedan had slowed down to make a left turn, but the drunk driver never took his foot off the gas and slammed right into the back of their car.
The impact crushed the trunk of their car and sent a car seat flying out of the back seat. The 4-year-old who was in the seat got ejected and he died from his injuries.
The highway patrol says it looks like the family did everything right, but the car seat couldn't handle an accident from behind with a pickup truck hitting them at 60 miles per hour.
Sgt. Craig Hinch, California Highway Patrol, says "They're going to take a look at was the car seat installed properly? Was it just the impact or what? But that's part of our investigation that we're going to take a look at."
Five other people in the car had to go to the hospital.
The other driver also went to the hospital. There was severe damage to the front of his pickup, which veered off the road after the accident.
He tried to run away from the accident scene, but the victim's family tracked him down and held him until police got there.