Jack, an active boy who could run and climb over gates, played in a puddle while his parents attended to some newly arrived cows in a pen next to the shed.
The parents then placed him in an enclosed room in the shed so he would be safe while they went about other tasks.
Mr Tatham was not far off and could hear Jack "nattering" to himself happily - but when he returned, he found the room empty.
He then noticed the cows were "spooked" and saw his son lying on the concrete pad inside the enclosure with cows.
Jack was unresponsive and clearly suffering from a head injury with swelling to his left eye.
An ambulance took Jack to Invercargill Hospital before he was transferred to the neurology ward at Dunedin Hospital.
Doctors concluded Jack could not survive his injuries and he died in his parents' arms the following day.
Ms Tatham today said she was happy to be cleared of any blame over his death.
"We're just trying to remember all the good things about him, not what happened."
She described Jack as a typical almost 2-year-old who "liked to go out on the farm with dad - he was a daddy's boy".
"He was always smiley and happy - his little smile would light up the room. Even if you were having a bad day, if he smiled or laughed or anything like that, it would make you feel so much better."
Jack was active and outgoing and would climb anything he could.
"You couldn't sit down and have a cup of coffee because he'd be off doing something."
Jack, who was named after his grandfather, left behind an older sister - and four months ago, a little sister was born.
"It wasn't long after the tragedy that I actually found out I was pregnant. So she's sort of helped in a way ... she was our special gift afterwards."
In his findings, Coroner David Crerar said it was clear Jack had opened the door to the shed on his own and went almost directly to where the cows were settling.
Jack was used to cows, having helped his mother feed them, so he would have been unable to differentiate the "skittish" cattle from calves.
"He wandered in amongst the cows and either slipped, or was knocked, to the ground. One of the cows stood on his head creating the fatal injury."
Coroner Crerar said while the parents were remiss for not supervising Jack the whole time, they had left him in a room where they knew he would be safe and were only seconds away from being able to stop him from entering the yard.
He found neither the parents nor the medical staff who tried to save him were to blame for Jack's death. No charges were laid.
"The tragic circumstances of the death do, however, serve as a lesson to all who have custody of vulnerable infants.
"A child who is unable to perform his, or her, own risk assessment needs to be constantly supervised by a responsible adult to ensure that no harm is created to them."
