Display all posts in main category and 1 subcategory

Question

I have defined a custom post type for creating galleries using WP built in gallery functionality.

register_post_type( 'my_gallery',
    array(
        'labels' => array(
            'name' => __( 'Gallery', 'my-child-theme' ),
            'singular_name' => __( 'Gallery', 'my-child-theme' )
        ),
        'public' => true,
        'supports' => array( 'title', 'editor', 'custom-fields', 'thumbnail' )
    )
);

Also created custom taxonomy for this specific CPT, with a main category 'Galleries', and a subcategory 'History':

-Galleries
    -History

I also have a template in child theme called taxonomy-galleries.php which works as it should, it displays all the posts in galleries, and in history too.

Now what I want is this: Display all the posts in main category, and display the subcategory history, paged with max 12 elements.

How would I do that?

Edit based on @mmm -s answer how to get all the elements in one array:

    $all_the_items = array();
    while ( have_posts() ) : the_post();
          $t = get_the_terms( $post->ID , 'my-taxonomy' );
                if ($t[0]->parent == 0){
                $properties = array(
                    'title' => get_the_title(),
                    'link' => get_the_permalink() );
                 $all_the_items[] = $properties;
}
endwhile; wp_reset_query();
    $o = get_queried_object();
            $children = get_term_children($o->term_id, $o->taxonomy);
            foreach ($children as $child) {
                $this_term = get_term($child);
                $properties = array(
                    'title' => $this_term->name,
    'link' => site_url('/' . $this_term->taxonomy . '/' . $this_term->slug . '/')
                );
    }

Answer 1

to get the children of the queried object, you can use this code is the file taxonomy-galleries.php

$o = get_queried_object();
$children = get_term_children($o->term_id, $o->taxonomy);

$children is an array of term identifiers then you just need to loop over this.