The fire happened near 1500 N. Beale Road on Thursday around 2:30 p.m.
The Yuba County Sheriff-Coroner's Office is working to identify the bodies and determine a cause of death.
At this time it is not known what caused the fire.
Neighbors say they heard "loud boom sounds" and when they came outside they saw the trailer on fire.
They didn't know anyone was inside the home until more law enforcement started arriving at the scene.
One neighbor told FOX40 the 4-year-old boy was living in the home with his father along with at least three other people.
This investigation is ongoing. Stay with FOX40 for updates.