I recently did something similar. I did this in an acf/save_post action. What I needed to do was to get the post status of the post being saved, then I used https://codex.wordpress.org/Function_Reference/get_children to get all the child posts, loop over them and alter the post status of each to the same as the parent.
In my case I also needed to add actions on wp_trash_post, untrash_post, before_delete_post that basically did something similar because I wanted child posts to be trashed/untrashed/deleted when the parent post was.
