I'm terrified I'm going to lock my son and my keys in the car together. So, when I'm putting my son in the car or getting him out, I put my keys on the roof of the car. I can put everything else in the car along with him (my purse or any shopping bags), but the entire time I keep the keys on the roof.
Of course, I drive a car, so the roof is pretty easy to get to. My sister-in-law drives an SUV, so she puts her keys on the hood.
I, too, am afraid of locking my keys in the car with the kids. I've attached a carabiner to my keychain so I can hang my keys on my purse strap when I need hands free. Another benefit: I keep them clipped to my purse when not in use, so can see at a glance if I've got my keys with me when grabbing my purse to head out the door.
How do you keep from locking your keys in the car?
Related: Keychain hacks