Are you literally asking about how the feature performs if there is a child hiding under the car?

Maybe not children, but in my city it's not uncommon to find cats and other small animals sleeping under cars during winter.

Yes. Because if there aren't any undercarriage sensors this will eventually lead to deaths.

And what prevents you from climbing into your car and doing the same thing? Do you check under your car every time before you drive off? I know I certainly don't and never have.

You might notice legs sticking out from under the car, and your getting in is going to cause a fair bit of alerting noise - doors opening and closing, you moving around. Without even a motor to start, the Tesla will just start moving.