A three-year-old girl suffered first-degree burns after sitting in corrosive cleaning fluid at a swimming lesson, a court heard.
The chemicals were spilt by maintenance staff at a teaching pool run by First Strokes Swim Schools in Stanway near Colchester.
The company admitted three breaches of the Health and Safety at Work Act at Chelmsford Magistrates' Court.
A district judge fined the firm £10,500 and ordered it to pay £2,350 costs.
The girl was sitting at the edge of the pool waiting for her lesson to start on 22 May last year when she began to complain of pain in her leg, the court was told.
When her mother took her to Colchester General Hospital, doctors said she had suffered first and second-degree burns.
First Strokes admitted a contractor had undertaken maintenance work earlier in the day using the company's own supply of sodium hypochlorite to treat the pool water.
A small amount of the chemical spilt on to the side of the pool, the court heard.
Prosecutors said the company had failed to assess the pool water treatment and cleaning chemicals used at the site, and failed to supervise contractors properly.
The company also admitted it had failed to comply with two improvement notices for its lack of hazardous substance control assessments, and to not having a safety system for dosing the pool by hand.
Mike Lilley, a councillor at Colchester Borough Council, who brought the case, said: "The public should be reassured that we take a very dim view of any business that fails to put health and safety first and whose actions lead to personal injury."