Officials say Logan Cox died Sunday morning at Carolinas Medical Center in Charlotte, North Carolina.
Teresa Clevinger told 911 operators her daughter had been watching the boy last Wednesday but dozed off while at the family's home in Buford. Clevinger said her daughter slept for about 30 minutes.
The Lancaster County sheriff's office says Logan suffered severe heat stroke after he climbed into a car at the family's home in Buford and became trapped. Buford is in South Carolina, near the North Carolina line.
The 911 call was obtained by WSOC-TV. Authorities haven't said if anyone will be charged in the boy's death.