Alexia Walenkaki died when a rotting playground swing collapsed on her in Mile End park in Tower Hamlets.
An inquest found poplar wood was wrongly used to make the equipment instead of oak, and the council had missed a safety check on the playground.
The council pleaded guilty to breaching Section 3(1) of the Health and Safety at Work Etc. Act 1974. They were fined £330,000 and ordered to pay costs of £6,204 at Westminster Magistrates Court.
The court heard that on 17 July 2015, Alexia Walenkaki was swinging on a rope attached at one end to a wooden post, when the play equipment gave way.
The post snapped at its base causing the wooden structure to collapse on top of her. She sustained fatal head injuries.
An investigation by the Health and Safety Executive (HSE) found that London Borough of Tower Hamlets Council had failed to ensure that an annual playground safety check was carried out.
Investigators also found the post was made from wood that was unsuitable and had decayed.
The local authority had previously implemented a system of inspections to ensure that play equipment was safe to use.
However, the play equipment at Mile End Park had not been inspected by a playground inspector since September 2013.
If the equipment had been inspected and tested for signs of rot, the risk may have been identified and appropriate action taken to remove and replace the equipment.
Speaking after her inquest Alexia’s mother, Vida Kwotuah, said: “It is clear to me now that there were many failings here. I am truly disappointed to learn how chaotic and disorganised management was within the council, which no doubt led to the missed annual inspection in 2014.
“Because of these failings I have lost my little girl.”
Ms Kwotuah said the death, on July 17, 2015 had “shook the foundations of our family”, and she was forced to relive the “horrible” incident during the two week-long inquest.
London Borough of Tower Hamlets Council pleaded guilty to breaching Section 3(1) of the Health and Safety at Work Etc. Act 1974. They were fined £330,000 and ordered to pay costs of £6,204.
Speaking after the hearing, HSE inspector Stephron Baker Holmes said: “Those who provide play equipment should ensure it is safe for children. The lack of a suitable playground inspection in the period leading up to this incident has resulted in tragic consequences.”