How to Display child post on his parent post with thumbnail and content in WordPress

Question

Just to check I understood your question, you want to display child posts under each parent post?

In that case you have to write one more WP_Query inside your while loop, with parent as current post in loop.

Something like:

        if ($query->have_posts()) :
            ?>
            <?php while ($query->have_posts()) : $query->the_post(); 

?> 
                <div class="col-md-3 col-sm-6 wow fadeInRight">
                    <div class="products_cat">
                        <div class="img_cls"> 
                            <?php
                               if (has_post_thumbnail()) { 
                                   the_post_thumbnail('full');
                               }?>
                        </div>
                        <div class="clearfix"></div>
                        <h2 class="title"><a href="https://wordpress.stackexchange.com/questions/255628/<?php the_permalink(); ?>"><?php the_title(); ?></h2>
                    </div>
                </div>
            <?php
                             endwhile;
            wp_reset_postdata();
            ?>

Edit:

Make a file single-products.php and add

$child_query = new WP_Query(array('post_type' => 'products', 'post_parent' => get_the_ID(), 'posts_per_page' => '-1'));
                           while ($child_query ->have_posts()) : $child_query ->the_post();
?> 
                <div class="col-md-3 col-sm-6 wow fadeInRight">
                    <div class="products_cat">
                        <div class="img_cls"> 
                            <?php
                               if (has_post_thumbnail()) { 
                                   the_post_thumbnail('full');
                               }?>
                        </div>
                        <div class="clearfix"></div>
                        <h2 class="title"><a href="https://wordpress.stackexchange.com/questions/255628/<?php the_permalink(); ?>"><?php the_title(); ?></h2>
                    </div>
                </div>
            <?php
                           endwhile;

Please check syntax errors and add necessary code which haven’t specified. Sorry I lost track of indentations after so many edits.