Firefighters initially found the bodies of the two adults and later discovered the children - an 11-year-old girl, a 7-year-old boy, and a 6-year-old girl.
A statement from the Riverside County Sheriff's Department said the victims' names will not be released until the bodies are positively identified. The Riverside County coroner's office is expected to perform autopsies Monday.
Investigators were looking into the cause of the blaze, which did not damage any other homes. The fire was extinguished by 25 firefighters around 6:30 a.m.
Hours after the blaze was extinguished, onlookers stood at every corner and watched fire authorities interview neighbors.
"I'm still grieving from this," said Jim Bernard, the mobile home park's manager. "I was up this morning helping them put it out."