Stepping through a foreach loop – adding +1 to the end of it so next time it loops it starts on item 2

Use the offset argument, and only get 1 result:

global $my_page_offset;

if(!isset($my_page_offset))
  $my_page_offset = 0;

$pages = get_pages(array(
  'child_of'    => $post->ID, 
  'sort_column' => 'menu_order', 
  'offset'      => $my_page_offset, 
  'number'      => 1,
));

$my_page_offset++;

...

I’m assuming here that you don’t have much control over these loops, because you’re doing them multiple times. This is why the global variable…

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Last try:

After var infowindow; add this:

<?php

$pages = get_pages(array('child_of' => $post->ID, 'sort_column' => 'menu_order'));
$data = array();
foreach($pages as $post){
  setup_postdata($post);
  $fields = get_fields(); 
  $data[] = '<p>'.$fields->company_name.'</p>';
}
wp_reset_query();

// the js array
echo 'var marker_data = ["'.implode('","', $data).'"];';
?>

Now infowindow.setContent should be:

infowindow.setContent(marker_data[i]);