Auto display first subpage content on parent page

Do you mean just show a static page?

That would just be done in Admin>> Settings>> Reading

No I want to load in automatic the content of the first subpage when i click on the parent page.
Example:

MAIN MENU: A B C

Page A has 3 subpages: a b c

When I click on main menu A link the content of subpage a is load in automatic and display.

I’m not sure on that one, sorry

ok this one shoul be easy. if it is not it is a fail.

I am looking for a way to implement this functionality myself.

i had the same problem and after a few google searches i found the solution here:

https://www.wprecipes.com/wordpress-page-template-to-redirect-to-first-child-page

<?php
if (have_posts()) {
  while (have_posts()) {
    the_post();
    if($post->post_parent){
        $my_subpage = new WP_Query(
                                  array('post_type' => 'page',
                                        'post_parent' => $post->ID,
                                        'post_status' => 'publish'));
        while($my_subpage->have_posts()): $my_subpage->the_post();
          ///Now put your code here according to your design
        endwhile;
     }
  }
}

If solve, mark it as resolve