For me, I would do nothing if the kids were old enough to get out of the car themselves. I'd probably wait in my car and watch to make sure someone came out soon if there were a baby or toddler left in a car, unless it was warm or parked in the sun, in which case I'd call 911.
I do leave my kids in the car for very short trips where I can park within visual distance, and only if both kids are there, because the 11-year-old can easily get herself and the younger one out of the car if it were to overheat. And never in the summer or if the car is parked in the sun.
I don't leave the younger one (4) alone in the car because she can't get out of her car seat alone (she's still in a convertible seat and the button is hard to press, which is good in most situations but not if she had to escape.) Also, even if she could get out of her seat, she'd probably get scared in the car alone.