Overwriting a Shortcode

A child theme’s functions.php file is loaded before the parent theme’s, so the behavior you’re seeing is correct.

Try adding your shortcode on a later action instead:

function wpa_add_child_shortcodes(){
    add_shortcode( 'jo_customers_testimonials_slider', 'jo_customers_testimonials_slider_with_thumbnail' );
}
add_action( 'after_setup_theme', 'wpa_add_child_shortcodes' );