Child theme css not loading when declared in parent functions

Because you’re using a subfolder path that starts with a /.

The official developer doc for add_editor_style has an example of how to add a stylesheet from a subfolder in the comments:

 * Registers an editor stylesheet in a sub-directory.
function add_editor_styles_sub_dir() {
    add_editor_style( trailingslashit( get_template_directory_uri() ) . 'css/editor-style.css' );
add_action( 'after_setup_theme', 'add_editor_styles_sub_dir' );