Seven-year-old Conley Thompson died when he slipped into a plastic pipe on a building site, his grandmother has said.
Conley's body was found on a building site in Worsbrough, near Barnsley, yesterday morning.
South Yorkshire Police said officers had checked the building site after Conley was reported missing on Sunday night.
"The construction site, where Conley was sadly discovered by construction workers this morning, was checked by officers during the hours of darkness and found to be securely locked with a large perimeter fence," a spokesman said.
"The investigation continues into the cause of Conley's death and we are working with the Health and Safety Executive as part of that investigation."
Conley's grandmother Deborah Fraser went to the site to pay tribute to her grandson and said she was "shell-shocked" by his death.
She said: "As a person he was a lovely little lad, boisterous, got into anything and everything but his smile could light up a room.
"It's not even sunken in now that he's gone. I'm stood next to the place where he died and I'm thinking 'what the hell were you doing Conley in a place like this?'"
Speaking about how Conley got trapped in the pipe, she said: "He slipped. You know it was raining, he slipped and went straight down. I hate to think what his little mind was going through.
"His brain must have been all over the place trying to get out."
Conley vanished after playing with friends in Locke Park, a mile away from the site and about two miles away from his home in Barnsley, police said.
He had gone to play at about 6pm on Sunday and was due to return home around two hours later, but never turned up.
People laid flowers and teddies at the construction site throughout the day.
Mrs Fraser said: "He's been known to go in there (the construction site) before and we've told him not to go in. It was raining and we just expected him to come home. (His mother) is devastated, we all are. We will all miss him, he was a lovely little lad.
"We all knew he had gone missing, but when you actually see it on the news and realise it's your grandson that's been found, you're shellshocked.
"He loved dinosaurs, he loved Ninja Turtles, he was just a typical lad. He like drawing and loved his PlayStation. He loved his life. He was a happy little chap."
South Yorkshire Police are still yet to confirm the body found at the site is that of Conley. Officers are understood to be treating the death as a "tragic accident".
Peter Taylor, manager of Security Guards UK, said the company was no longer employed on the site, although its signs remained.
"We were employed on the site until July 16. During the time we did report on numerous occasions kids trying to get into the site and various trespassers," he said.
"If a security guard was on site then he would have spotted it and would have prevented it from happening. We are very sorry for the family's loss."
He described the site as "high risk", with building materials and diggers inside.
A book of condolences is being opened at the nearby St Thomas' Church in Worsbrough.