They were all in an open carriage on the Water Chute ride at Coney Beach fairground, Porthcawl, Mid Glamorgan.
Timothy Morgan, of Roath, Cardiff, was thrown clear and suffered head injuries. He died at the scene. His father Christopher, 46, and brother David, 15, were taken to the Princess of Wales hospital, Bridgend, with face, chest and shoulder injuries. Last night Mr Morgan's condition was serious.
Also in the carriage were Daniel Preston, 12, Dominic Evans, 12, and Raymond Verral, 17, all from Cardiff. They were treated in hospital.
The ride's buggies are pulled up a 50ft-high track before turning and descending into a deep pool.
The fitting was one of several arch- shaped metal hoops with ornamental lights which straddle the track. It struck the carriage as it was on the downhill section.
David James, 23, who saw the accident, said: 'Some of the overhead lighting came down on to the track just as the ride was gathering momentum. The boy appeared to be hit.' Other bystanders said the carriage had been knocked off its tracks.
The amusement park had just opened for the first day of the summer season despite storms which had battered the South Wales coast overnight. Fifteen miles away, Barry Island funfair remained closed after the helter-skelter was demolished and several rides were damaged by winds which had reached 100mph.
South Wales police said of the fatal accident: 'We do not know if the weather was a factor . . . that will be something which Health and Safety Executive inspectors will have to
Last night Pat Evans, the fairground owner, said it was his first accident in 50 years in the business.
In June 1993 three people were thrown out of a funfair ride at Newcastle upon Tyne. Two months later 12 people were injured at Blackpool when a metal bar on the roller coaster snapped off. In 1972 three children died and 16 others were hurt on the Big Dipper at Battersea fun fair in south London.