Get Single child category of a Parent

if ( ! function_exists( 'themeplate_get_child_category' ) ) {
    function themeplate_get_child_category( $id ) {
        $taxonomy = 'category';
        global $post;

        // Get the term IDs assigned to post.
        $post_terms = wp_get_object_terms( $post->ID, $taxonomy, array( 'fields' => 'ids' ) );

        // Separator between links.
        $separator = ', ';

        if ( ! empty( $post_terms ) && ! is_wp_error( $post_terms ) ) {

            $term_ids = implode( ',' , $post_terms );

            $terms = wp_list_categories( array(
                'title_li' => '',
                'child_of'  => $id,
                'style'    => 'none',
                'echo'     => false,
                'taxonomy' => $taxonomy,
                'include'  => $term_ids
            ) );

            $terms = rtrim( trim( str_replace( '<br />',  $separator, $terms ) ), $separator );

            // Display post categories.
            return '<div class="child-cat">' . $terms . '</div>';
        }
    }
}