The crash happened at about 8:45 p.m. Sheriff's investigators say the Honda Civic the children were traveling in crossed the center divider and slammed head-on into a Nissan Sentra on the 3500 block of San Gabriel River Parkway.
Gilbert Enriquez, 12, and Braulio Duque, 13, both of Whittier, died after being ejected from the vehicle, and the driver, 22-year-old Effron Velasquez, was also killed. Family members said the children were not wearing seatbelts.
Albert Delgado said he was driving in his truck when he heard the crash ahead of him.
"I just heard a bang," he said. "I saw bodies everywhere, so I was trying to help the kids. I stayed with one; he was breathing, but by the time they came, he stopped breathing."
A woman and a friend of the two boys, identified as 13-year-old Humberto Flores, were also in the Honda. They were rushed to a local hospital.
"He's able to talk. He just has a little bit of road rash, a little gash," said Flores' godfather. "He doesn't remember a lot. I was trying not to make him talk too much because he was badly injured."
The female passenger with them was hospitalized with critical injuries.
Rene Ayala, Flores' cousin, said they were all at Flores' home for a pool party celebrating their elementary school graduation. Velasquez was driving a couple of the boys home at the time of the crash.
Friends and family gathered Friday night at a growing memorial at the scene of the crash.
"It's devastating. I mean, it's the first child, the first grandchild, it's just something that you don't want anyone to go through," said Enriquez's aunt, Elsie Ruiz.
Two women who were in the Nissan were rushed to a local hospital. They were listed in stable condition Friday.
The cause of the crash remains under investigation. Deputies say speed may have been a factor. So far, there are no indications that drugs or alcohol played a role in the crash.