Police were in the area for a separate incident when they turned on the fire at 2:19 p.m., officials said.
Bystanders tried to enter the house to save the family, but were turned back by heat and smoke. When firefighters arrived, the flames were through the roof, emergency responders said.
Firefighters were able to enter the house through a window and soon found two children who were handed over to paramedics. The children died on the spot.
Firefighters continued to search and found another child and an adult woman who had already died, firefighters said.
Investigators are now working to find out the cause of the fire. Firefighters said most of the damage is at the back of the mobile home and spreads forward.