Hi Dona,
This is we have defined to manage it. Yes you make exact the same structure of parent theme while creating child theme and create two file style.css and functions.php and enqueue the responsive.css,
We have designed our theme this way. So if you are unable to customise the code then you need to hire a developer to fix it.
Hi Dona,
You simply add your responsive code inside dashboard -> appearance -> theme options -> design options -> custom css and customise the code. This is the easiest method.