One of them, a six-year-old girl, is currently in critical condition and placed under intensive care at Pamela Youde Nethersole Eastern Hospital. She developed a fever, cough and runny nose on Thursday.
Two of her classmates also showed mild flu-like symptoms recently, though neither required hospitalisation.
Meanwhile, a three-year-old girl and a 15-year-old boy remain in serious condition. All household contacts of the three patients are currently asymptomatic.
Edwin Tsui, Controller of the CHP, said overall flu activity has stayed high since the summer influenza season began.
He noted that 16 severe paediatric cases have been recorded since the end of June, including one fatality, with two other patients currently in critical condition.
Tsui warned that more outbreaks are expected following the start of the new academic year, urging both parents and schools to stay vigilant and maintain preventive measures.
Members of the public are also urged to get vaccinated when the annual Seasonal Influenza Vaccination Programmes begin on September 17.