Pinellas County Sheriff Bob Gulatieri said 13-day-old Kathleen Bridget Steele was pronounced dead at St Petersburg General Hospital.
However, investigators believe she died earlier, inside the van where her mother, Kathleen Steele, had left the children for 38 minutes while at a phone repair shop.
“It was one of the worst things I’ve ever seen,” the sheriff said.
Investigators say Steele went into the mobile phone store at 1.50pm last Monday.
The six-year-old boy told investigators that the baby started fussing and he tried to calm her.
But deputies said the boy later used a doll to show how he slammed the infant’s head into the minivan’s ceiling, dropping her on the floor, flipping her over, and pummelling her.
“The baby was beaten and traumatised,” the sheriff said. “There was gross swelling in her face. Her skull was cracked in numerous places.”