Question

Example :

About
--- technical
--- medical
--- historical
--- geographical
--- political

how to create a function like this ?

function get_child_pages_by_parent_title($title)
{
    // the code goes here
}

and calling it like this which will return me an array full of objects.

$children = get_child_pages_by_parent_title('About');

Answer 1

You can use this, it does work on page ID rather then title, if you really need page title, I can fix it, but ID is more stable.

<?php
function get_child_pages_by_parent_title($pageId,$limit = -1)
{
    // needed to use $post
    global $post;
    // used to store the result
    $pages = array();

    // What to select
    $args = array(
        'post_type' => 'page',
        'post_parent' => $pageId,
        'posts_per_page' => $limit
    );
    $the_query = new WP_Query( $args );

    while ( $the_query->have_posts() ) {
        $the_query->the_post();
        $pages[] = $post;
    }
    wp_reset_postdata();
    return $pages;
}
$result = get_child_pages_by_parent_title(12);
?>

It’s all documented here:
http://codex.wordpress.org/Class_Reference/WP_Query