How to get children/attachments in header?

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From Codex: get_children – examples:

If you just want to get or display attachments, it’s probably a little easier to use get_posts() instead.

So you might go on and do that, the link in above cite leads to an example.

Besides that change your code to:

global $post;
// array with parameters
$args = array(
    'post_parent' => $post->ID,
    'post_type' => 'attachment',
    'orderby' => 'menu_order', 
    'order' => 'ASC',
    'numberposts' => -1,  
    'post_mime_type' => 'image'
);
$images = get_children( $args );
if ( $images ) {
    foreach ( $images as $img_id => $img ) {
        echo wp_get_attachment_image( $img_id, 'full' );
    }
}

Main difference being that you have to do:

global $post;

because you’re outside the loop.