10 year olds know how to unlock the door and get out before they overheat or die of hypothermia!! That "rule" is for BABIES. A one week old...or even a 6 month old...can NOT unlock the door and escape a car that is too hot or too cold.
And yes, the police would be called if you "went to check the mail and something happened"...but only for HELP...NOT to arrest you for endangerment or neglect!! A healthy, "normal", 10 year old, in most states, CAN be left unattended for a couple hours.
Now, of course, if a child has special needs, the laws vary based on their specific need(s).
Also, why are you sending your child alone to the bus stop 20 minutes early? If you're able to, watch for the bus, as long as you don't have a mile long driveway, send them out when it arrives. If you can't, time the bus. Get your child a watch and make her keep note of the schedule for a week, then send her out only 4 or 5 minutes before the bus is due. Keep in contact with the school to see if/when they have a new driver (who may be unfamiliar with the route, causing the bus to be early or late). Our local school district sends letters home in these cases, so we never have to wonder, other close school districts send emails or texts, etc. Maybe even bring that up at the next PTA/PTO meeting about the school implementing something like that. A generic form with "Bus Number ____ will have a substitute driver __________. Please be aware as your child's bus may come earlier or later than normal during this time." The school can easily print several of these "forms" out on a single sheet of paper, cut them out, fill in the bus number and the date/dates the normal driver will be out, then just give it to the kids who are on that specific bus. That simple phrase will fit on a typical Word document 17 times per page. In Microsoft publisher, it will fit anywhere from 10 to 24 times, depending on how big the font is, what font you use and if you do single or double columns. Mine like to be out 15 minutes early, but, they're sisters, riding the same bus, as well as many other kids at the stop with them if one or the other has to stay home sick.