If you want to override file, you should copy it into Child theme and modify, but please remember to keep the same directory structure.
If you i.e. want to modify Fest/css/tablet.css file, please create css directory in your Fest-child then copy tablet.css file there and add your modifications, if that won't work, please send me a PM with backend and FTP access to your website, then I'll be able to check the issue exactly.