The first curse word my first child ever uttered was when he was three years old. He was riding with his mother (shotgun in the "carseat of protection +5") when the garbage truck in front of them lost a load of garbage cans.
When my son saw the cans go airborne towards them, he grabbed his seatbar and yelled "Shit!!!!!" at the top of his lungs.
Usually my wife goes into a volley of oaths in such situations, but she was swerving and braking through the obstacles, too busy to say a word.
My later joke to her was "you must have been so proud...
"
Interestingly, a couple of years later, he and I were headed home late at night in a rainstorm when we were broadsided by an RV running a red light. A few 360s and curb hops later - his only words were "Did we just get hit?" I said, "Yes, and I should pull off the road before we get hit again" (the Z car was totaled but was good for about 20 more feet - fortunately for my son the impact was mostly near the front wheel rather than the passenger door).