What they should have done
To me, if you accept the simple premise that we ought to make some effort to keep alive, it is just a rock stupid thing to keep your family in a place that you know is radioactively contaminated and where you are 100% certain that they will eventually die. Look, these people survived for months after the blast. Further, they don't have any radiation burns and don't even begin to get sick or lose their hair until late in the movie, which suggests that they are dieing because of contaminated water/food rather than as a result of radiation exposure from the initial blast.
The US is a big place. There are places in it that will be several hundreds of miles away from any blast. They've got a buddy who has a HF radio capable of making contacts across the world. We know what the Russians will bomb and there are projections for where the fallout will be minimal, given the prevailing west to east wind currents. People would know these things in 1983, especially hams, many of whom are nutjob survivalists anyway. There would be places that they could go, and it's reasonable to assume that they would know of them.
They have another buddy who runs a gas station and is willing to give them free gas. They could fill up the tank and some extra containers with gas, pile as much canned food as they can get into the car and drive up the coast to Oregon, which should be almost entirely fallout free. What do they do when they get there? If there are no survivor's communities or government camps, they could hunt, fish, pick berries, start a garden -the things that people have done to survive for as long as we've been around. No, it won't be a pleasant life, and, yes, there's a pretty good chance that they'd die anyway. But you have to play the odds in life, and Russian roulette is a better bet than a firing squad.
My point is that you don't just lie down and die, which is I think the reason that this film left a bad taste in my mouth. The BBC movie "Threads" was almost a horror story in it's relentless worst case scenario presentation of what a nuclear strike would be like, but at least the characters in it kept moving. This movie was like Family Circle on nihilism.