An adventurous four-year-old boy climbed on the rubble at the Gaumont Cinema demolition site at North End.
He either fell or a piece of brickwork fell on him.
City coroner, PD Childs, told an inquest that the brickwork on his chest made it impossible for him to breathe and died from traumatic asphyxia.
The boy, of Pitcroft Road, was playing with friends when he wandered off into the site.
He was found by police after his mother reported him missing.