She said they all began to breathe more easily when they moved further away from the beach, but took the 10-year-old girls to a walk-in clinic to be checked over by medical staff.
She added: “It’s not what you expect when you go for a day out to the beach.”
Training manager, Mark Wray, who was at the beach with his wife, said he had noticed a few children coughing as they came out of the sea, but assumed they had swallowed some water.
He said officials later began patrolling the beach and starting to tape off some areas, before “a procession of emergency vehicles, including ambulances, fire appliances, police cars and other unmarked vehicles with blue flashing lights started heading towards the scene”.
He said the beach was “packed with hundreds of families enjoying the record-breaking temperatures”.
Essex Police said that while the cause was investigated, people were advised not to go into the sea.
East of England Ambulance Service said people should wash themselves down if they were in the water, change their clothes and drink fresh water.
Anyone with further concerns is advised to call the NHS on 111.
A spokeswoman for the Maritime and Coastguard Agency said: “At around 2.30pm on Sunday afternoon, HM Coastguard received reports of beach swimmers suffering from a possible respiratory irritant causing people to cough at Frinton and Walton-on-the-Naze on the Essex coast.