How to disable alphabetical sorting page

If you look at get_pages() in the Code Reference you’ll notice two parameters: sort_order and sort_column.

So, the following example will give you child pages list sorted by menu_order:

$pages = get_pages( array( 
    'child_of'    => $post->ID,
    'post_type'   => 'page',
    'post_status' => 'publish',
    'sort_column' => 'menu_order',
    'sort_order'  => 'desc',
) );

You can change menu order manually

or using a plugin, e. g. Simple Page Ordering.