I do believe that they should have somehow showed or mentioned the fates of the Vito's other grandchildren in Part III. But it would have had little impact in the overall arc of Michael's story. It would have been the same as the few lines they added about Hagen's death, just referencing some previous characters to bring some closure.

Now if they had decided to do a Part IV, maybe you could work in some of the legit children to fill in the script with familiar names and make them part of a story, post-III.

I always thought a Sonny, Jr vs. Vincent power struggle could have been interesting.