we want to create child themes that will only override or add a few things like blog files

Hi. You can make override of com_content with yor own file names (excluding "default" file names) and menu item type using main Joomla means.

I believe that the child theme option is unlikely to have a reason with any framework-template, but not only with Helix. Since the framework-template have complex architecture, which can lead to problems with the implementation of child theme functionality. And we will get years of beta testing and fixing problems :).

This is good only for simple templates.

When this option appeared in Cassiopea, I also asked this question here. But having tested Cassiopea and understanding how it works, I came to the conclusion that this is hardly possible in Helix and the like. Because Helix heart is a system HelixUltimate-plugin. Simple templates as Cassiopea do not have the like.