Seven of the eight children, all aged under 16, have been discharged following the incident in Woking, Surrey.
One boy was kept in for overnight observations but suffered injuries “not thought to be significant”, police said.
Surrey Police declared a “major incident” at Woking Fireworks after multiple children fell from the “giant inflatable slide” at about 7.30pm on Saturday.
Those taken to London hospitals were treated at St George’s Hospital and King’s College Hospital, while one was treated at St Peter’s Hospital in Chertsey, Surrey, South East Coast Ambulance said.
One witness said he was concerned earlier that evening when he saw up to 40 children playing on the “flimsy” slide at once.
The investigation into the incident is now with the Health and Safety Executive, police said.
In a statement on Sunday, Surrey Police said: “Following last night’s evacuation of Woking Park after children fell from an inflatable slide during fireworks night celebrations, we are pleased to update that seven of the eight victims were discharged overnight.
“The eighth victim was kept in overnight for observations, but his injuries are not thought to be significant.
“The investigation will now sit with the Health and Safety Executive.”
Police have appealed for anyone with footage of what happened to contact them.
Andy Datson, 23, said he saw up to 40 children playing on slide, which he estimated was about 30ft tall at its highest point.
“We had been walking past the slide earlier in the night and said it looked unsafe. It looked pretty flimsy to say the least,” he said.
“There were far too many kids on it. It didn’t look like it could hold that many people.”
The journalist, who lives in Guildford, Surrey, said that he noticed the flashing lights of ambulances after walking off with friends.
“I immediately thought, ‘that’s going to be the slide’,” he said.
“It looked like there were too many children on it and the side area had given way.
“If, heaven forbid, they fell from the top I would say it’s at least 25-30ft tall.”
Oliver Trimble told the BBC that the evacuation was announced over the event’s loudspeakers shortly before 8pm, when the fireworks were due to begin.
He said he saw victims being treated by medics, adding: “It was quite a horrendous sight to see.”
“This is one of Woking’s biggest events of the year, thousands of people come to see the fireworks,” he said.
Local musician Anna Neale said she was among the hundreds of people who were evacuated from the park, which had been packed with families with young children.
The 36-year-old said: “We saw all the blue lights and saw the air ambulance arriving. We all evacuated very calmly. Everyone was just shocked.”
She had planned to take her daughters, aged 10 and six, on the giant slide after the fireworks.
Ms Neale said: “I am very grateful my children didn’t go on it, let’s just put it that way.”
Woking District Rotary Club, the event’s organisers, said it was “shocked and distressed” by the collapse and said it would meet to “agree next steps”.
Following news of the children’s conditions, Woking Fireworks tweeted: “We are really pleased for all concerned to hear that the injuries are not as serious as first feared. We are meeting shortly to agree next steps.”
The park will remain closed into Sunday as police investigate the cause of the incident, the club added.
A Health and Safety Executive spokeswoman said “initial inquiries” were being made.
A Woking Borough Council spokesman said: “We will be working with the police and relevant authorities to understand and explain what happened.”