// parent's theme class
class MyThemeClass{

    public function add_filter(){
        add_action("excerpt_length", array($this, "filter_excerpt_length"));
    }

    public function filter_excerpt_length($length){
        return 50;
    }
}
$desktop_theme = MyThemeClass;

// child theme's class
class MyChildThemeClass extends MyThemeClass{

    public function add_filter(){
        add_action("excerpt_length", array($this, "filter_excerpt_length"));
    }

    public function filter_excerpt_length($length){
        return 20;
    }
}
$child_theme = MyChildThemeClass;