Carson Dean Cheney was holding onto the headstone Thursday when some metal connecting it to the pedestal broke, said Park City police Capt. Phil Kirk.
Some of the children being photographed were not being responsive, so Carson tried to help the photographer - his father - by pretending to be a leprechaun and making them laugh, said Curtis Morley, a family friend. Morley said the boy went behind a tombstone and was playfully poking his head out from behind it when it fell on him.
"Carson passed away while trying to make others smile," Morley said.
Carson was just about to enter kindergarten, loved to ride his bike and was "full of life," said his grandmother, Geri Gibbs.
"There's still so much disbelief and sorrow and anguish," she said.
"We just keep waiting for the door to open up and Carson to come through, a happy little boy."
She said it took three men to pull the slab off the boy, and rescuers "did everything they could possibly do."
The child suffered injuries to his head, chest and abdomen and was taken to the nearby Park City Medical Center, where he died. Authorities were still investigating Friday.
Gibbs said the boy and his family were visiting from Lehi, about an hour away. Morley, who works with the boy's father, Zac Cheney, at a professional services firm in Salt Lake City, said Zac Cheney does photography in his spare time and was shooting portraits at the cemetery because of its extensive landscaping.
Park City Police Chief Wade Carpenter said the coarse stone at the Glenwood Cemetery in Park City, about 4 inches thick, marked the grave of someone who died in the 1800s.
Bruce Erickson, president of the Glenwood Cemetery Association, said the private, five-acre cemetery around the corner from Park City Mountain Resort was founded by a society of silver miners in 1885, and many of the tombstones are at least 100 years old. The cemetery is open to the public and still accepts burials of people connected to the mining society.
Erickson said no funerals were held there Thursday.
New burials happen about once a year, he said, and families are responsible for maintaining the headstones. Erickson said the cemetery likely will be closed through the weekend.
A funeral for the boy is set for Tuesday. Morley said a memorial fund has been set up at Zions Bank.
Associated Press writer Lynn DeBruin in Salt Lake City contributed to this report.