Ok, I struggle with this. I have 7 month old twins and a 3 year old that I drop off at Mom's Day Out twice a week. It takes me less than a minute to drop him off in his class room, but it takes me 10 minutes to unload my babies into a stroller, walk everyone inside and turn around and load everyone into the car again. Not to mention that they are usually asleep, so taking them out of the car wakes them up.
I have left them in the car once, when it was raining and cold. The car was right in front of the church building and it was boxed in by two other cars (which I did on purpose). I was in the building less than a minute (I ran ) and I still hated leaving them in the car. I'm not sure if I will do it again, but I felt dragging them out in the rain would have been worse.
It really is a tough one, and sometimes there are no good options when you have more than one baby.
And to those of you who would reply "Then you should just stay home" know that most of the time I do. And that sucks too.