Which do you think would be worse; seeing a child alone driving a car or a child playing with a loaded gun?

Seeing the child is bad enough.

Nobody should be playing with a gun - loaded or otherwise.

When I was a child, I used to have nightmares that I was alone in my parent's van ... and it would start moving, and I had to unbuckle my seatbelt, get behind the steering wheel, and avoid crashing into various obstacles.

I suppose it would depend on the age of the child.  A toddler could pick up a gun, but wouldn't be able to drive a car.  

WHAT? Assume a loaded gun is loaded? Preposterous!

Given that a young child wouldn't be able to engage the gears, an automatic stays in park and a manual either remains in neutral or, if parked in gear (mine always is), it wouldn't start at all. Either way, either it doesn't move or rolls slowly. In the absence of a steep hill, max damage might be a dented fender.
Guns kill people. So the gun would be infinitely worse.