Leaving the kids unattended anywhere is not a safe thing to do. However, I guess everyone's idea of what "unattended" means is a little different. I leave my kids locked in the car to run in and pay for gas if I can see them. I can't always see my kids at all times in the house, but my mama-radar is on and I can hear them. I always take advantage when I have an adult passenger, to run into the grocery store or the bank.
I could not fathom, leaving them to go inside and watch t.v and relax. That would simply be impossible.
I have had to run in and grab some forgotten item after finally getting the kids loaded up.
My older son can get out of his booster seat, and the car, if he needs to. I have never left them for longer than about 30 seconds tops except when I lived in a rural location and the baby was asleep and I would leave the car door wide open and hang out in the yard about 15 ft away from the car.
Anything else would be negligent in my opinion.