You need to check if the page have parent to display the child page template or if he doesnt have to display the parent page template. If $post->post_parent is equal to 0 thats mean that he doesnt have a parent.
while ( have_posts() ) : the_post();
$template = ($post->post_parent == 0) ? 'page' : 'child-page';
get_template_part( 'template-parts/page/content' , $template );
endwhile; // End of the loop.