"I can not believe what I just witnessed on Gorleston beach", Kayla-Ann Weaver told the Daily Mirror.
Police said only that they were called to the scene on the Lower Esplanade around 11.15am following reports a "child had been thrown from a bouncy castle".
The young girl was taken to the James Paget Hospital but died.
The East of England Ambulance service received more than a dozen calls just after 11am.
An inflatable at a beach in Norwich exploded and threw a child 20ft in the air, witnesses say..
The girl, who authorities didn't identify, was critically injured when she was flung from the bouncy castle in Gorleston, a seaside town on the coast of Norfolk in east England., the Guardian reported.
A post-mortem is due to take place into her death on Monday afternoon following the tragedy on Gorleston beach in Norfolk on Sunday morning.
"Several of our teams were dispatched, with the first on scene in four minutes", they said in a statement.
She was rushed to hospital but was sadly pronounced dead. A man gave her cardiopulmonary resuscitation until an ambulance arrived.
Superintendent Roger Wiltshire, of Norfolk Police, said the incident was "just unimaginable".
Robert Halfon, the Conservative MP for Harlow, led calls for a government review into the rules governing the use of inflatable castles.
The area of beach where the incident happened has now been cordoned off as police launched a joint investigation with health and safety officials.
Summer Grant was playing inside the inflatable at a fair in Essex in March 2016 when it blew away from its moorings, bouncing for 300 metres before hitting a tree.
Fairground workers William Thurston, 29, and his 26-year-old wife Shelby were jailed for three years in June over the "entirely preventable" death of Summer Grant.