It’s weird for me regarding this issue. You’ve been modified the right file. Can you please provide me an example of the problem that you get in there? I will check it directly on my side.
For the child theme, you just need to move the modified file into the child theme. For example, if you edit this file jnews/fragment/header/element/desktop/logo.php then it will become something like this jnews-child/fragment/header/element/desktop/logo.php (use same folder structure).