[101Volts]

The point is: Little kids including ones who aren't closely related who may end up getting to a car while I'm not looking. I've seen at least five Rescue 911 episodes (Which are re-enactments of actual events though they did dramatize them a bit) in which little kids played around with car transmissions while the cars were on:

One nearly rammed a pregnant woman and a grocery store employee pushed her out of the way and was pinned between that car and the one he was loading up. (He recovered if I remember right.)
One nearly went into cross-traffic and a post-man stopped the car.
One DID go onto the road even though the parking brake was on (it failed) and a police man stopped it by blocking it with the car he was in.
One nearly went in traffic but a woman stopped it though she was run over in the process. (She recovered.)
I'm sure I can remember the fifth one too.
There's also a fictional movie in which a baby or toddler did that and a teenager on a skateboard crawled in through the sun-roof to stop the car; It would've been hit by a train otherwise.

I don't have kids around yet (I'm not even married) but I like the thought of having this in cars. That's while remembering to supervise them, The interlock is an extra provision. Thanks for the wheel-chocks suggestion, I do have some that can be used whether I stick this in said cars or not. However, I can think of situations where wheel-chocks aren't viable.