I have this same scenario every day only I usually have my 3 yr old DD fall asleep on the car too.
I get my DS walked to the kiss & go zone and I pick him up from there, or I simply wake the girls and walk them in. I do have a double pram I could take but it's not worth the bother for 5-10 minutes. I wouldn't ever consider leaving my girls in the car let alone unattended at home.