Category page with posts from 2 different childcategories

To include posts from one or more specific categories, you can use and define category__in parameter in your WordPress query. Like this, here is an example.

$args = array(
    'category__in' => array( 11, 15 ),
);

$my_query = new WP_Query( $args );

if ( $my_query->have_posts() ) :
    while ( $my_query->have_posts() ) : $my_query->the_post();

        get_template_part( 'content', get_post_format() );

    endwhile;
endif;

wp_reset_postdata();

Where 11, 15 are the category ids of the categories that you want to include posts from. You can add as many categories as you want. You just have to add comma separated ids of those categories.