I used to leave mine asleep in the car on the drive behind a locked gate and visible from where I was in the house. I would, if necessary, leave them in the car while paying for petrol rather than try and wrangle them across a busy forecourt. In reality, I could count on one hand the number of times that happened thanks to paying at the pump. I would not leave them in a car, in a public place, out of sight. I see that as a very different thing.
I sincerely doubt it was only 30 seconds. It was clearly long enough for the TA to notice and get concerned. You could very easily have had the teacher wanting to urgently talk to you, or your DC fall over and need attention, or another parent wanting to talk to you, or any number of things that would have held you up.
How long do you think would be too long to leave him there? I mean from your own description you were concerned enough to make the time as short as possible and felt the need to sprint. So you obviously didn't want to leave him more than a minute or two? If you're scoffing at the idea that anything bad could happen then what do you think could happen in 10 minutes that couldn't happen in 2?
When bad things happen they happen very fast.
Don't be sure there aren't things happening about the other family you refer to either. More than likely stuff is happening but it's just not visible to you. Staff at school can't force the parents to be responsible, unfortunately. My job would be much less depressing if that were the case.
I bet if you'd said you'd left a dog in the car this thread would be full of YABUs.
Do you live too far away from school to be able to walk? It's so much easier using a buggy for the school run and let the baby sleep in there. No need to leave it anywhere then. Not much help if you're too far away though.