Good on you for phoning the NSPCC, concernedmum.
Pie, I don't mean leaving on the air con on in the parked car - to do that you'd have to keep the engine on and as you say, it would be dangerous to leave the keys in the ingintion with the engine ticking over.
But if the air con had been on just before the engine was stopped, the interior might have been cool. In this case, opening the windows of the parked car would have been counterproductive because all you'd be doing is letting the cool air out.
Aloha, I see the scenario you were faced with was more serious than children being left for a minute or two. And if you could see them sweating inside that's horrible.
For those, like me, who sometimes leave children in a parked car for a minute or two - please can I say do take out the cigarette lighter accessory. I forgot to do this, and my oldest son, when he was age 6 fiddled with it and got a very nasty burn on his finger.