Display post of current parent category, and in right hand side show child category post

To display posts only from current category, without posts from sub-categories, use pre_get_posts action hook to set 'include_children' => FALSE in tax_query parameter.

add_action( 'pre_get_posts', 'se342950_no_child_term_posts' );

function se342950_no_child_term_posts( $query )
{
    if ( is_admin() || ! $query->is_main_query() || ! $query->is_category() )
        return;

    $tax_obj = $query->get_queried_object();
    $q_tax = [
        'taxonomy' => $tax_obj->taxonomy,
        'terms'    => $tax_obj->term_id,
        'include_children' => FALSE,
    ];
    $query->query_vars['tax_query'][] = $q_tax;
}

---

Posts from only the parent category you can get with the function get_posts(), in which also include_children should be set to FALSE.

// in category.php "get_queried_object()" returns WP_Term object
$tax_obj = get_queried_object();
$posts_from_parent = [];
if ( $tax_obj->parent != 0 )
{
    $args = [
        'post_type' => 'post',
        'tax_query' => [
            [
                'taxonomy' => $tax_obj->taxonomy,
                'terms'    => $tax_obj->parent,
                'include_children' => FALSE,
            ]
        ]
    ];
    $posts_from_parent = get_posts( $args );
}

---

Update

Display child category posts.

category.php

?>
<div class="col-sm-12 brand-review-global">
   <h3> Brands Review  </h3>  <!-- here I want to show post of B -->
<?php
global $post;

//
// get child category
$tax_obj = get_queried_object();
$children = get_terms([
    'taxonomy' => $tax_obj->taxonomy,
    'parent' => $tax_obj->term_id,
    'fields' => 'ids',
]);
if ( is_array($children) && count($children) )
{
    $child_cat_id = array_shift( $children );
    //
    // get posts from child category
    $args = [
        'post_type' => 'post',
        'tax_query' => [
            [
                'taxonomy' => $tax_obj->taxonomy,
                'terms'    => $child_cat_id,
                'include_children' => FALSE,
            ]
        ]
    ];
    $posts_from_child = get_posts( $args );

    //
    // display
    foreach( $posts_from_child as $post)
    {
        setup_postdata( $post );

        ?>
            <div class="brand-review">
                <a href="https://wordpress.stackexchange.com/questions/342950/<?php the_permalink();?>">
                    <?php the_post_thumbnail(); ?>
                </a>
                <!-- other things: the_title(), the_excerpt(), ... -->
            </div>
        <?php

    }
    wp_reset_postdata();

}
else
{
    echo 'No data to display';
}
?>
</div>
<?php