The woman, believed to be a holidaymaker in her early 50s, is thought to have tried to rescue her son after he got into difficulty at Northcott Mouth beach, near Bude, in North Cornwall.
A member of the public also rushed into the water to try to save the pair but also had to be rescued from the strong current.
The tragedy – just 24 hours after lifeguard cover on the beach ended – was witnessed by the woman's other son who was on the shore.
Devon and Cornwall Police were caring for the two boys, thought to be aged 11 and 13, as officers last night tried to contact next of kin. Frantic 999 calls were made at about 2pm yesterday as beachgoers watched the tragedy unfold.
Falmouth Coastguards alerted RNLI lifeguards on nearby Summerleaze beach, called out the inshore lifeboat from Bude, scrambled a search and rescue helicopter and sent its own cliff rescue team.
"Two lifeguards on duty heard the call and responded immediately to the scene where they retrieved a female casualty from the water," a spokesman for the RNLI said.
"The lifeguards carried out cardiopulmonary resuscitation until the arrival of a search and rescue helicopter. The casualty was then airlifted and transferred to North Devon District Hospital.
"Meanwhile, the three volunteer crew on the Bude inshore lifeboat who had also responded, were tasked to rescue a child from the water and a member of the public who had gone into the sea to try and help the youngster.
"Both were taken onboard the lifeboat and delivered into the beach where they were met by the local coastguard unit."
He added: "This is a tragic situation and the thoughts of the whole RNLI team are with the woman's family and friends at this sad time."
The woman was flown to North Devon District Hospital by the rescue helicopter from RMB Chivenor. Despite the efforts of her rescuers and medical staff she was later pronounced dead.
A spokesman for Devon and Cornwall Police said: "The woman – believed to be in her early 50s – appears to have got in to trouble while in the water at Northcott Mouth Beach at around 2pm. She and one of her children were removed from the water with the help of the coastguard, lifeguards and other beachgoers.
"She was airlifted to North Devon District Hospital by a Royal Naval helicopter where she was later pronounced dead. Next-of-kin are being informed at this time.
"She is believed to have been with her two children at the time. The children are unharmed."
A spokesman for Falmouth Coastguard yesterday praised the swift response of the emergency services.
"Everyone was at the scene very quickly," he said. "Lifeguards did incredibly well to be there in five minutes, the inshore lifeboat was there in 15 minutes and the helicopter was already in the air. We had an incredible response from all of the units involved."
Lifeguard cover on the rural beach, which was busy with visitors yesterday, ran from July 7 until Sunday.
The RNLI spokesman said: "Northcott beach is a peak season beach and therefore has RNLI lifeguard cover for the two-month high season summer period from July 7 to September 2. The period of lifeguard cover at any beach is decided after the charity carries out risk assessments on the beaches at the request of the local authority or private beach owner.
"This includes the numbers and types of beach users, numbers and type of incidents, natural hazards, topography and proximity to other rescue services and makes recommendations on the level of safety cover on the beach, including the season dates, number of lifeguards and the type of rescue equipment.
"Information on the locations, dates and times of local lifeguard cover is displayed in the area."
