Auto update cart after quantity change. You trigger the click, but the button doesn't have enough time to become enabled, so that is why, by the time you click the second time the button becomes enabled. Remove the "disabled" propriety before triggering the click. This should be added in **footer.php** of your child theme

WooCommerce Auto update cart after quantity change.php

<?php if (is_cart()) {    ?> 
    <script type="text/javascript">   
      // Each time quantity in the cart changes, trigger update 
      // cart option
      jQuery('div.woocommerce').on('change', '.qty', function(){
            // Make sure the button is enabled before triggering the event
            // otherwise this won't work.
            jQuery("[name='update_cart']").prop("disabled", false);
                jQuery("[name='update_cart']").trigger("click"); 
      });
  </script> 
<?php  } ?>

This Gist comes from this stackoverflow thread.
https://wordpress.stackexchange.com/questions/271341/auto-update-cart-after-quantity-change