An inflatable trampoline appeared to be "overfilled" with air before it exploded and threw a little girl to her death, an inquest heard.
Ava-May Littleboy was playing on the attraction when it burst on the beach at Gorleston-on-Sea in Norfolk on July 1 2018.
Witnesses said she was sent soaring into the air, higher than the height of a house, before landing on her face on the sand.
The three-year-old, from Lower Somersham in Suffolk, died in hospital of a head injury.
Giselle Johnson, director of Johnsons Funfair Limited, trading as Bounce About, said that on the day of the accident she thought the trampoline was "overfilled".
"I did notice that the blue bit was overfilled so I went back to the shed and turned it off," she said.
"In my mind, the equipment had a lot of air."
She said that she had put up a Minions-themed bouncy castle at the site that morning and her husband Curt Johnson put up an inflatable slide.
Norfolk's senior coroner Jacqueline Lake, reading from a statement that Mrs Johnson gave after the accident, said that Mr Johnson had "helped to set up the trampoline but left to go to Great Yarmouth while it was inflating".
Ms Lake asked Mrs Johnson which pieces of equipment two child workers were involved in putting up.
After she was given a warning that she may incriminate herself, Mrs Johnson declined to answer.
The children, a boy who was under the age of 16 at the time, and an under-18 girl, cannot be identified due to reporting restrictions.
They were referred to as Mr A and Miss B.
Mrs Johnson declined to answer a question about whether it was Mr A's "responsibility" to let her know when the trampoline was "at a point wherethe fan should be turned off".
She said she trained staff and Mr A had worked for Bounce About for around one month at the time.
"He has done two days training," said Mrs Johnson, who was assisted in court by a Portuguese interpreter.
"He knows everything."
"He was fantastic there, he knows everything how we work."
Mrs Johnson said she was in a shed at the site looking for a checklist at the time of the explosion.
Ms Lake said that Mrs Johnson made a statement regarding Miss B after the accident.
Miss B "couldn't explain why" she allowed Ava-May onto the inflatable trampoline "while it was connected to the fan and knew it should never be done while it's still connected to the fan", according to Mrs Johnson's statement, which was read by Ms Lake.
Mrs Johnson told the inquest that Miss B came to see her in the days after the accident.
"She came to my house to get her wages," she said.
"She was in bits, I gave her a hug, we cried together.
"She didn't know what she had done."
The inquest heard that the inflatable trampoline had been inspected five days before it exploded.
Mrs Johnson said it was the first time that equipment inspector Henry Rundle had visited and usually it was his father.
She said that, during the inspection visit on June 26, Mr Rundle told her "everything was fine".
Ms Lake said that Mr Rundle's report, dated July 2 2018, recorded that "not all available tie-downs were in use, beds were found to be firm".
It added that for all new devices "manufacturer's information must be supplied as required", Ms Lake said.
The inquest is being heard by a jury in Norwich.