Getting child category id (help with improving my code)

Managed to solve this issue myself

   <?php
$idObj = get_category_by_slug('winners-2018'); 
$id = $idObj->term_id;
$args = array(
  'orderby' => 'name',
  'parent' => $id,
  'taxonomy' => 'category',
  'hide_empty' => 1 
  );
$categories = get_categories( $args );


        foreach($categories as $category){
        $cat_archive = get_category ($category);
        //var_dump($cat_archive);
        global $wp_query; 

        $args = array_merge( $wp_query->query_vars,array(       
            'paged'=>$paged,    
            'posts_per_page'=>10,
            'post_type'   => 'winners',
            'category_name' => $cat_archive->slug,
        ));

        ?>
        <div class="outer">
            <div class="inner">             
                <div class="inside">
                    <h1><?php echo $cat_archive->name; ?></h1>

                </div>          
                <div class="left-content winners_fullwidth <?php if(is_category()) { echo $cat_archive->slug;} ?>">
                    <div class="wrap-content clearfix">    
                        <?php
                        $cposts = get_posts(array(
                            'orderby' => 'title',
                            'order' => 'ASC',
                            'posts_per_page'=>10,
                            'post_type'   => 'winners',
                            'category_name' => $cat_archive->slug,
                        ));
                        foreach ( $cposts as $post ) : setup_postdata( $post ); ?>
                        <div class="press-box">                                
                        <?php if ( has_post_thumbnail() ) { ?>  
                            <div class="art-image">
                                <a href="<?php echo get_permalink(); ?>" title="<?php the_title();?>"><?php the_post_thumbnail( 'square-blog' ); ?></a> 
                            </div>
                            <?php } ?>                                
                            <div class="art-right">                                   
                                <h2><a class="perma" href="<?php echo get_permalink(); ?>"><?php the_title();?></a></h2>
                                <?php the_excerpt();?>
                            </div> 
                        </div>
                    <?php endforeach;?>
                    <?php wp_reset_query(); ?>  
                    </div><!-- wrap-content -->
                </div><!-- left-content -->
            </div>  
            <?php } ?>
Solution was

    $idObj = get_category_by_slug('winners-2018'); 
$id = $idObj->term_id;
$args = array(
  'orderby' => 'name',
  'parent' => $id,
  'taxonomy' => 'category',
  'hide_empty' => 1 
  );
$categories = get_categories( $args );


        foreach($categories as $category){
        $cat_archive = get_category ($category);
        //var_dump($cat_archive);
        global $wp_query; 

        $args = array_merge( $wp_query->query_vars,array(       
            'paged'=>$paged,    
            'posts_per_page'=>10,
            'post_type'   => 'winners',
            'category_name' => $cat_archive->slug,
        ));

Starting from your parent category slug you can use the following code to get the child categories:

$parent = get_category_by_slug('winners-2018');

$catz = get_categories(
    array( 'parent' => $parent->cat_ID )
);

foreach ($catz as $cat_archive) {
    // No need for get_category(); because get_categories() already returns category objects
    ...
}