At about 5:30 p.m. Sunday, police and firefighters were called to Birch and Third streets near the Brentwood Neighborhood Church to investigate a report of an explosion.
When they arrived, officers found a group of children who had allegedly set off the fireworks, police said.
Among them were two 12-year-olds, a boy and a girl, who were seriously hurt and required immediate medical attention.
The girl was taken to a hospital with injuries to her hands and face and the boy, who was airlifted to a different hospital, had major injuries to his hands and "burns over a large portion of his body," according to a Brentwood police news release Monday.
None of the injuries were considered life threatening.
Officers investigating the incident discovered more fireworks at a home in Oakley and are working with the Contra Costa County District Attorney's Office to determine if any charges will be filed.