The blaze was reported at 10233 Mission Gorge Road just before 2 p.m. The cause of the fire was not known.
When crews arrived, at least two units on the first floor of the complex were fully involved.
One child suffered second- and possibly third-degree burns. A firefighter suffered minor burns to his arm, according to a fire department spokesman.
An occupant of the unit where the fire is believed to have started told CBS News 8 he heard two loud pops before realizing there was a fire. He and his roommate escaped through a window.
Investigators remained on scene working to determine the cause of the blaze, a Heartland Fire & Rescue dispatcher said. It was not immediately clear how many units were affected.
Power was shut off to at least one building in the complex, and it was possible service would remain out for as long as a week.