If I can't see them at all times then I don't leave them. As tempting as it is I would rather wake DS for kinder pickup/dropoff rather than leave him as I can't see the car from inside kinder. Not only is it a safety issue to leave him but I also think that if he woke up and found me gone he would be scared and probably start crying/screaming, even if I was only gone 2mins lol!
If my DS happens to be asleep I will often ask another mother to bring my DD out from kinder so I can stay in the car with DS and I do the same for her as well if her little one is sleeping.