Rawai Mayor Aroon Solos said local rescue workers were informed of the incident at 4:32pm.
An ambulance arrived at the scene, near the Nai Harn Baan-Bua Development, north of Nai Harn Lake, at 4:38pm.
A large number of onlookers had gathered around two boys, aged 4-5. Both boys were unconscious and having difficulty breathing, Mr Aroon said.
Rescue workers provided oxygen to the youngsters and requested support through the 1669 emergency hotline.
While waiting for an ambulance from Chalong Hospital to arrive, one of the boys stopped breathing. Rescue workers performed CPR until the medical team from Chalong Hospital arrived at 4:48pm, Mr Aroon said.
Medical personnel continued performing CPR until they reached Chalong Hospital at 5:08pm, but the child was still unconscious, he added.
At 6pm, officers reported that CPR had been stopped because "there is a pulse", but the boy was still unconscious.
The boy was still in the emergency room at Chalong Hospital, but officers were preparing to move him to Vachira Phuket Hospital for further treatment.