Chessington World of Adventures has been fined £150,000 for safety breaches after a four-year-old girl suffered head injuries in a 14ft (4.2m) fall.
The girl, from Kent, suffered a bleed on the brain and fractured skull when she fell from the queue for the Tomb Blaster ride in 2012.
Guildford Crown Court heard that water dripping from a roof had rotted fence posts lining the queue to the ride.
The theme park said in a statement it was an isolated incident.
The Surrey theme park pleaded guilty to the breaches on Friday after the Health and Safety Executive (HSE) accused it of failing to properly inspect and maintain the fence lining the queue.
The court was told on the day the girl was injured, another family further up in the queue had to grab their daughter by her coat to stop her falling through the fence.
As well as head injuries, the victim, who cannot be named for legal reasons, also suffered broken ribs.
She was in hospital for a month and the HSE said she still needed extensive rehabilitation treatment and specialist support.
'Disastrous and horrifying'
Its investigation found the fence palings had white and brown rot and had not been painted with a preservative.
There was also evidence that numerous palings had been re-fixed over the years.
Chessington World of Adventures Operations Ltd, of Poole, Dorset, was also ordered to pay £21,614 costs.
The HSE said the girl suffered life-changing injuries.
"This was a disastrous and horrifying incident for the child and her parents," said inspector Karen Morris.
"Chessington had insufficient measures in place to prevent or control the risk of falling from the raised walkway."
The theme park said it regarded the health and safety of its guests and employees as its number one priority.
In a statement, it said: "What happened to the young child was an isolated incident which we deeply regret, and from the outset we have put in place a package of support for the child and the family which will continue for as long as is required."
It has since installed metal safety fencing along the queue line, it added.