A happy Sunday family outing exploded into a terrifying nightmare for supermodel HEIDI KLUM, three of her kids and hunky bodyguard-turned-boyfriend MARTIN KIRSTEN!

They were in LA watching “Frankenweenie” when the screen abruptly went blank in the pitch-dark theater – and the crowd instantly panicked, fearing a copycat scenario of the Aurora, Colo., movie theater massacre that occurred when a gunman opened fire, killing and wounding dozens.

Reports My SpyWitness: “Heidi immediately grabbed one of her children, shouted to Martin to grab the other two, and the couple pushed their way into the crowded aisle as people screamed and raced for the exits!” Just as Heidi, Martin and kids reached the exit doors, the lights snapped back on abruptly – and they were nearly knocked flat by a flying squad of paramedics that came charging into the theater.

Explained my source: “It turned out that a woman, sitting two rows in front of Heidi, had suffered a heart attack, gone unconscious and slumped over, triggering the emergency.”

Breathing a sigh of relief, Heidi hustled her brood out of the theater – turning up her nose at management’s offer of free passes for another flick.