A fairground worker told an inquest an inflatable trampoline exploded and threw a little girl to her death, just as she had turned away to check if customers were allowed to be on it.
Ava-May Littleboy was playing on the attraction when it burst on the beach at Gorleston, Norfolk, on 1 July 2018.
The three-year-old, from Lower Somersham in Suffolk, died of a head injury.
The coroner told the worker what happened "was not your responsibility".
The teenager, called Miss B due to reporting restrictions, told the hearing in Norwich she was the funfair worker who put Ava-May on the trampoline.
She said while the girl was playing she went to check with Giselle Johnson, the director of Johnsons Funfair Limited, if she was allowed to be on there.
She said she had earlier tried to check and saw someone give a thumbs up, which she "assumed" was Mrs Johnson.
"I wanted to check as I'm not a mistake-y sort of person," said Miss B, who was 17 at the time. "By the time I had turned around it had... exploded."
Miss B, giving evidence via Skype, said the trampoline's owner, Curt Johnson, left the site while the trampoline was "partly inflated".
"I told him he could trust us to put it all together and he could go and set the other business up [in Great Yarmouth]," she said.
She said she knew nobody was allowed on to the trampoline if the fan was connected to it.
Miss B said she went to see Mr and Mrs Johnson at their house in the days after the incident, "upset, apologising".
"I don't know why I didn't 100% check about putting anyone on the trampoline or why I didn't check the pump was still in. They said 'it's not your fault', but you blame yourself anyway."
Norfolk's senior coroner Jacqueline Lake told Miss B: "What happened on that day was not your responsibility. I want to say that as I gather you have been upset by it.
"Thanks for your help on the day, as I gather you did try to save Ava-May."
Inspection engineer Henry Rundle, who inspected the inflatable five days before it exploded, told the inquest Mr Johnson phoned him after the incident "asking to say that his devices were safe".
"I thought it was quite unusual given what had just happened," said Mr Rundle. "It's something which they obviously weren't."
He said when he inspected the beds of the trampoline, they were "firm" which was a "good thing".
"If the beds aren't firm the structure isn't working as it should so there's a possibility of walls falling down," he said, adding: "Beds found to be firm is a good thing for a bouncy castle."
The inquest continues.