It seems the bug [Plugin: JSON API] get_page_index returns only five pages is only partly fixed, indeed when looping on the children of a page it returns only 5 pages.
I temporarily fixed it in json-api/singletons/introspector.php , function attach_child_posts, adding numberpost key in the array below.
$wp_children = get_posts(array(
'post_type' => $post->type,
'post_parent' => $post->id,
'order' => 'ASC',
'orderby' => 'menu_order',
'numberposts' => 100
But I am sure it would require a more in-depth fix.
Thanks for the great plugin anyway :-)