The second happened 13 hours later on U.S. Highway 1, where a 63-year-old Bath woman was killed by a motorist running a red light.
There have been 20 fatal crashes in Aiken County this year, authorities said.
Cassidy Heath, 8, was riding in a truck driven by her father, Kenneth Heath, who ran a stopsign outside Aiken about 10:30 p.m. and struck an oncoming SUV, according to the South Carolina Highway Patrol.
Emergency personnel sent to the intersection of East Pine Log and Wagener roads said Cassidy died at the scene. The other six people in the truck were taken to Medical College of Georgia Hospital with injuries ranging from minor to severe.
Troopers say Heath had a "strong odor of alcohol," and that he admitted to drinking a beer prior to the wreck. Blood was drawn at the hospital for a toxicology test, but those results were not complete Saturday, said Lance Cpl. Josef Robinson with SCHP.
Also under investigation is whether Cassidy was riding in the cab or the bed of the truck, Robinson said.
"Alcohol and failure to wear seat belt appear to be factors resulting in this collision and the death of the child," the highway patrol said in a news release. No charges have been filed against Heath.
The other people injured in the Heath vehicle were Michelle McNeal, Jordan Steverson, Trenton Heath, Elizabeth Morris and Jay Rogers, a report said.
Cassidy's mother, Tiffany Stephens, said by phone Saturday that she didn't want to be interviewed.
The driver of the other vehicle, Teresa Dennis, and her three children were all wearing seat belts and suffered minor injuries, according to the SCHP.
On Saturday, "inattention" to a red light at the intersection of Duncan Road and U.S. Highway 1 led to a fatal two-car wreck, Aiken County Coroner Tim Carlton said.
Patricia Schar, 63, of Bath, was driving west on Duncan Road in her Buick LeSabre when she was struck by a Jeep Cherokee as she entered the intersection, Carlton said. She was taken to Medical College of Georgia Hospital where she died from her injuries.
The Cherokee's driver, 35-year-old Gabriel Weaver of Hephzibah, who failed to stop at the red light, was not wearing a seat belt, but it was unclear whether he suffered any injuries.