If he can give his house, his car, his money, etc. to his children, I don't see why he can't give them his copyright.
It would be very unfortunate if an author worked for 10 years (not unreasonable) on a fantastic novel, published it, it became a bestseller, and then he died one week later, and it instantly became public domain.
The publishing company that gave him an advance and spent money on marketing the book would lose money (and so they would probably never sign a deal with an old man or someone with bad health again), and the author's children would not benefit from their parent's 10 years of work. If his children died then 1 month later, surely their children (the author's grandchildren) should be able to benefit. It seems that length of time is more relevant than number of generations.
You touch on a good point. I think our copyright term in the US is far, far too long. But, until the copyright term is changed, it is the law and we should respect that, especially since most of the audiance on this site makes their living through IP of one kind or another.
Also, for this particular book, remember that the coauthor is alive, the primary author died fairly recently(1988) and this book wasn't published all that long ago(1985). So even if you agree with me that copyright terms are far too long, this one would probably still fall within one reduced to a reasonable time frame (I personally think the right number is around 25 years from first publication).
The fact that you consider 1988 to be "fairly recently" shows how badly US copyright laws and lobbying efforts have skewed people's perceptions. In 1988, there was no Facebook, Youtube, Google, Amazon, or web, cellphones were virtually unheard of, many of the people on this forum were unborn, the Berlin wall was standing and Michael Jackson was at the top of the world. It was a long, long time ago.
The justification for intellectual "property" is to encourage creators to work that will eventually belong to the public. I cannot believe that an author's decision to write a book at that time was influenced by whether or not could continue extracting profits in today's world.
The original copyright term of 14 years might have been defensible for books in that time, but extensions are very, very hard to justify in terms of the benefits to society.
And I think you may be closer to the mark with 14 years than I was with 25, though some franchises continue to be active longer than 14 years. The Simpsons for instance are still producing new episodes after roughly 23 years.
Either way, 1988 is extremely recent in comparison with the way the current copyright laws are written and many other books that are still under copyright after multiple decades. I may disagree with the law as it is now, but until we manage to get it changed, it is the law.
Feynman also has two children. Surely he is allowed to have his children benefit from his work after his death.