make admin.css in my child theme

The get_template_directory_uri() function gets the URL to the parent theme. The equivalent function for child themes is get_stylesheet_directory_uri().

However these days the proper function to use to get the URL for a file in a child theme is get_theme_file_uri():

function wp245372_admin_enqueue_scripts() {
    wp_enqueue_style( 'my-admin-css', get_theme_file_uri( 'my-admin.css' ) );
}
add_action( 'admin_enqueue_scripts', 'wp245372_admin_enqueue_scripts' );