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.