How to compile SCSS correctly using WordPress Child Theme

I’m having a hard time understanding how to compile a child theme in WordPress. My main theme compiles using compass into an app.css file. I’ve created the standard style.css in a child theme folder. WordPress recognises this. I’ve created a scss file inside the child theme folder, I’ve then added it to the import list in the app.scss file in my main theme. Compass doesn’t throw an error when trying t import but Compass isn’t recognising changes in the child theme scss file though. Is this the correct set up? I’ve put the import after my general variables file so I can overwrite the variables in the child theme.