Show numbers of related post based on sub category and HIDE parent category

<?php       
$categories = get_the_category();
$child_cat = null;

foreach ($categories as $category) {
    if ($category->parent != 0) {
        $child_cat = $category;
        break; // Stop after finding the first subcategory
    }
}

if ($child_cat) {           
    $args = array(
        'cat' => $child_cat->term_id,
        'orderby' => 'rand',
        'posts_per_page' => 7, // Correct parameter
        'post__not_in' => array(get_the_ID())
    );
    $related = new WP_Query($args);
    if ($related->have_posts()) {
        echo '<div class="main_relp"><ul class="cur_rpost">';
        while ($related->have_posts()) {
            $related->the_post();
            echo '<li><div class="relp">';
            echo '<div class="img_relp">' . get_the_post_thumbnail() . '</div>';
            echo '<div class="title_relp">' . get_the_title() . '</div>';
            echo '</div></li>';
        }
        echo '</ul></div>';
        wp_reset_postdata();
    }
}
?>

Explanation

Removed if( 0 != $related->current_post ) to ensure all posts are displayed.
Changed showposts to posts_per_page for correct parameter usage.
This will now display the correct number of related posts based on the subcategory.

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