I didn't want to be a complete asshole, but nor was I inclined to drive off and leave them there, so I decided to give 5 minutes and then call the cops and ask them how long I should give 'em. A mother or a reasonable substitute came back out of a shop just as the deadline ticked over.
Mind you, leaving small children in an unlocked car is asking for disaster on many levels. Not just the someone wandering off with them problem, but the older of the children was certainly big enough to wiggle out of his car seat and either fool with the car brakes or wander off. But I wouldn't normally call the cops on those grounds, heatstroke I will.
Apart from that little mini drama, we had a nice day wandering around Filoli. The house is mostly just pleasant, in that stately home way, but the gardens are fabulous. For all the wedding planning folks, they hold weddings there, though it looked sized more for small weddings than large.