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.