The scaffolding had been erected by construction company Westdale Services, which pleaded guilty to breaching s 3(1) of the Health and Safety at Work Act over the incident on 6 May 2017.
Cardiff Magistrates' Court was told that two boys accessed the site at Southville Flats in Cwmbran. They were able to climb the ladder within the scaffolding by placing their feet either side of a ladder guard that did not cover the rungs of the ladder.
One boy went to the top platform of the scaffold and then climbed the uppermost ladder to a height of about 10 m. The ladder slipped, and the boy fell to the ground, sustaining life-changing injuries requiring multiple operations. He now has no bladder or bowel control and is only able to walk short distances.
An investigation by the Health and Safety Executive (HSE) found the security arrangements for preventing access to the scaffolding, especially by children from a nearby school, were inadequate.
Westdale Services has been fined £160,000 and ordered to pay £22,310 in costs.
Speaking after the hearing, HSE inspector Michael Batt commented: "A lot of thought must go into securing construction sites. Children do not perceive danger as adults do. The potential for unauthorised access to construction sites must be carefully risk assessed and effective controls put in place.
"This incident could have been prevented by removal of the ladder completely or installing an appropriately sized ladder guard to cover the full width of the rungs."