Like many of the other people who have already commented on your request, I would encourage you not to delete books from your list because they might be somewhat dark or complex. I remember reading Bradbury, Cordwainer Smith, Asimov, and Clarke when I was your sons' age, and appreciating the fact that the worlds and scenarios they created had ambiguities and shades of gray.
That said, I would recommend taking a look at a history of science fiction for suggestions. My own personal favorite is Brian Aldiss and David Wingrove's Trillion Year Spree: The History of Science Fiction - it does a great job of covering the classics of science fiction, while pointing out lesser-known authors. The book summaries and analyses should give you a good feel for whether a particular author would be of interest to your sons.
Thus mathematics may be defined as the subject in which we never know what we are talking about, nor whether what we are saying is true. -- Bertrand Russell