The fire at 310 Washington Ave. was reported around 6:20 p.m., and as fire crews arrived they were told a possible victim was trapped inside.
The first arriving engine found heavy fire and smoke coming from the front of the home.
Deputies from the Kern County Sheriff's Office were called to assist the fire department with crowd and traffic control.
After making entry to fight the fire from inside the home and to search for the possible victim, firefighters found the toddler deceased, according to fire department spokesman Corey Wilford.
It took crews just over 30 minutes to fully extinguish the fire, which is now under investigation.
Eyewitness News later learned there were a total of four children inside the home when the fire started.
Neighbors rushed to rescue the children but were not able to locate the boy, identified later by the coroner's office as 21-month-old Aiden McDonald. The search by neighbors was abandoned as fire crews arrived, because the heat was too intense.
"I tried to find the baby, but couldn't," said a tearful woman as she recounted what happened. She declined to give her name.
Aiden was found in a rear bedroom of the home.
Investigators said the fire was mainly contained to the first floor. Damage was estimated at $140,000, while the value of the structure and contents saved was put around $210,000.
Neighbors told Eyewitness News this was the second death in three days for the family. On Monday, the family's great-grandmother died after a long illness.