It's horrible that it seems the parents died before the children too. What an awful way to die, but totally understandable that in the moment they were more worried about retrieving the rental car and making it to LA on time than finding shelter for the long haul.
There's no "long haul" out there. I've never been in that part of the park, but from looking at the map I would say the only likely survival there would be to hunker down and push the button (the backcountry equivalent to calling 911--not an option back then.)
If that were not an option I would wait for nightfall, keep all lights and navigation, space blankets, sun umbrella and the jerry can of water, abandon everything else and try to hoof it. I would consider success unlikely. Given the supplies they had I would consider survival impossible.
It depends on the situation. If you have had a breakdown and have to hoof it (which normally shouldn't happen anyway--you should be prepared to hold out until you're reported overdue and they're looking for you) you could be in a situation where you know ahead is shorter than behind. That assumes you know where you are and what the roads around are like.
