Officers were called to Lower Esplanade, Gorleston-on-Sea, at about 11.15am on Sunday, Norfolk Constabulary said.
Lifeboat crews and nearby witnesses battled in vain to save the “seriously injured” youngster, who later died in the James Paget Hospital.
The East of England Ambulance Service said it received more than a dozen calls about the incident.
A spokeswoman said: “We attended the incident this morning at Gorleston beach, involving a patient using inflatable equipment. More than a dozen calls were received at just after 11am, and several people at the scene including the Royal National Lifeboat Institution (RNLI) were providing assistance to the patient.
“Several of our teams were dispatched, with the first on scene in four minutes. The young female was seriously injured and in cardiac arrest on our arrival, and was conveyed to James Paget Hospital. Sadly, despite all the of the efforts and interventions, she was pronounced deceased.
“We would like to thank everyone who rushed to respond the young patient, and did everything possible to give her treatment and care. Our thoughts are with the family at this time.”
Images of the scene show an area of the beach, with what appears to be crumpled canvas and abandoned deck chairs, cordoned off by police.
Uniformed officers are standing beside the fenced off area. There are also people in the background enjoying the hot weather.
Brandon Lewis, Conservative MP for Great Yarmouth, tweeted: “Absolutely tragic, thoughts are with family & friends, just so sad.”
A joint investigation between the Health and Safety Executive (HSE), local authority and police has been launched into the circumstances around the incident.
The child’s next of kin have been informed.