I get a lot of my Blu Ray information from Bluray.com, and it lists both as Region Free
https://www.blu-ray.com/movies/Dune-Blu-ray/106754/
https://www.blu-ray.com/movies/Children-of-Dune-Blu-ray/181608/
Also, the BluRays themselves show they are "Region ABC", which in Blu Ray terms is region Free
https://www.sony.com/electronics/support/articles/00029369
Here's a pic with the Region Code on the back of both cases:
The ABC Honeycomb shape means Region Free.
Finally, my Blu Ray player is just some random $55 Sony one I got from Wal-Mart on my last Birthday, its nothing special so I doubt it would play a Region locked disc even once. Also, I checked and the Blu Rays also work in my PS4, which (while the games are region free) definitely doesn't play DVDs or Blu Rays that are Region Locked.
So, I'd say that the discs are pretty much definitively All Region/Region Free, and Amazon might just have some generic listing for Blu Rays that are imports that all say they won't work because of region differences. Its probably worth doing a bit of research if there is something you really want, especially since a lot less Blu Rays are region locked then DVDs, which are almost all region locked.