Their father, Michael John Simpson, was hospitalized with life-threatening injuries, also with gunshot wounds, authorities said.
And their mother, Suzanna Brown Simpson, 35, who a school district official said was an active volunteer at the school, was charged with two counts of murder, one count of attempted murder and one count of possession of a weapon during a violent crime, according to warrants signed by authorities.
Thirteenth Circuit Solicitor Walt Wilkins said the case could qualify for the death penalty if a conviction is reached because it involves multiple homicides and also because it involves children under the age of 11.
Carly Simpson was 7 years old; Sawyer Simpson was 5 years old.
On what otherwise may have seemed like a perfect spring day in a community of pastures swaying with bright yellow dandelions, Dacusville was in mourning Tuesday.
Two yellow ribbons adorned the front door of the school, as secretaries went through the sad business of calling parents of the children's classmates.
Principal Michael Fleming was hardly able to talk about the children.
"Precious children, always smiling, seemed to enjoy school, just really sweet kids," was all he could say by late afternoon.
Life inside the school had gone on as normally as could be expected during the day, he said. Authorities had notified school officials of the tragedy but told them there was no need to lock the school down, he said.
They said the danger had passed.
Tim O'Connor, who lives across the street from the Simpsons, was one of the first to realize something terrible had happened.
"I had just got out of bed," he said. "I was sitting there watching TV and drinking my coffee, and I heard boom, boom, boom, boom, boom.
"At first it sounded like whenever somebody's dumping a dumpster. And then it kept going on."
He decided someone had had a car wreck.
He was right.
When authorities arrived around 6 a.m., they found Suzanna Simpson had been in a truck crash just outside the house and had injuries, officials said.
When they checked inside the house, they found the children and Simpson's husband, they said.
Sheriff Rick Clark said that he is focused on bringing justice for the victims and couldn't release many details about the case.
He said he didn't know what precipitated the pre-dawn violence. There had been no domestic violence calls from the home that authorities were aware of, he said.
"Every community, no matter how great, has its tragedies, and Pickens County has experienced another one today," he said.
"As always in Pickens County, we'll come together. We support our own, and we're here for the family of these victims, and we care for them very much."
Suzanna Simpson had no criminal record, as far as authorities had been able to determine.
The sheriff said he couldn't comment on her condition, although he said she is under guard at Greenville Memorial Hospital.
Nor would he answer whether her husband and the children were asleep when they were shot.
He also wouldn't say how the wreck happened, or whether authorities believe that the mother was trying to flee.
The ages of the victims made the job that much more emotionally intense for investigators, he said.
"No matter how tough a cop you are, no matter how many people you've prosecuted, doesn't matter who you are, when kids are involved it's a whole different situation," he said.
"We're going to be working with our deputies, too, because they're human, they came into this and we're going to make sure that they're all right, too."
The State Law Enforcement Division sent crime scene agents to assist in processing the scene, according to spokesman Thom Berry.
Glenn Abbott, who said he moved to the neighborhood within a few weeks of the time the Simpsons did nine years ago, described them as a "very nice couple."
"They were a typical, ordinary middle-class family, worked hard," he said. "I never dreamed that would be even possible."
It had been a tough day since hearing the news, he said.
"Lot of tears and a lot of prayers," he said, "praying for Michael to pull through."
The Simpsons lived in a modern-looking ranch house with a freshly mowed lawn and landscaping that included several magnolias.
Harvey Alverson, whose property adjoins theirs in a wooded lot across O'Shields Road, said, "As far as I know they were pretty good people. I just don't understand what happened to them."
He's seen seven fatal car crashes in his yard, along a curve in the road during the 41 years he's lived there, but never anything like this, he said.
James Brown, who has lived down the road just past the school for almost all of his 74 years, said the community recently has been stricken with burglaries - "people stealing lawnmowers and trailers, anything they can get money out of."
Otherwise, it's a very quiet neighborhood, he said.
He passed by the Simpson house often.
"I might see the lady come to the mailbox, but I couldn't say I really know them very good," he said. "Used to, you knew everybody who lived in every house everywhere. But now everybody works, you just don't socialize like you did 50 years ago."
Grief counselors will be at Dacusville Elementary today for students and staffers, school district spokesman John Eby said.
He described Suzanna Simpson as "a very active parent" at the school.
"It would be safe to say she was a model parent at the school," Eby said.
In a statement issued by the district, Carly's teacher, Nancy Zeigler, described her as "a very smart little girl who loved to help her fellow classmates. She was a leader in the classroom and loved by all."
Her brother's teacher, Jama Freeman, said "We will miss Sawyer's smile in our classroom."