Three children were still in hospital on Monday after a firework hurt 11 people at an organised Bonfire Night display on Saturday.
About 15,000 turned up for the organised display
A hospital spokesman said Bruno Tarrant, 10, was stable in a specialist unit after surgery for leg burns.
Two others, aged eight and 11, are also stable in Kettering General Hospital.
An investigation has begun into how the 11 were hurt when the firework shot more than 180 metres into the crowd at Wicksteed Park in Kettering, Northants.
No criminal offence
Bruno's grandparents said the youngster was expected to undergo a third operation on Tuesday.
Angela and Salvatore Stefanelli, of Regent Street, Kettering, said their daughter Luciana, of Morley Road, who was standing next to Bruno at the display also needed treatment for leg and back injuries.
They said she was at Bruno's bedside at Sheffield Children's Hospital, despite her injuries.
On Sunday Kimbolton, the Cambridge-based company which organised the event, apologised to the injured and their families and said it would review its own safety measures.
Environmental health officers from Kettering Borough Council are investigating after police found no criminal offence had been committed.
Martin Hammond, who is leading the investigation, said: "We need to sit down with Wicksteed Park and Kimbolton Fireworks to establish their view of what happened.
"Health and safety inspectors will be looking at the equipment and what happened there."
About 15,000 people had turned up to the fifth annual display hosted by the Kettering leisure park.
Seven minutes into the show, a firework went into the crowd where children were standing behind the cordon.