one last post per child cat

<?php

$parent_cat = get_term_by('name', 'Blog', 'category');
$child_cats = get_categories('child_of='.$parent_cat->term_id); //set the parent category
$child_cats_ids = array();
$counter = 0;
if( $child_cats ) foreach( $child_cats as $cat ) { $child_cats_ids[] = $cat->term_id; } //get array of child cat IDs

$child_cat_posts = get_posts( array( 'category__in' => $child_cats_ids, 'posts_per_page' => -1 ) );

if( $child_cat_posts ) foreach( $child_cat_posts as $post ) {
    foreach( get_the_category($child_cat_post->ID) as $post_cat ) {
        if( in_array( $post_cat->term_id, $child_cats_ids ) ) {
            setup_postdata($post);
            //start output of post//
            the_title; echo ' - '; the_date(); echo ' - ' . $post_cat->name; echo '<br />';
            //end of post output//
            $key = array_search($post_cat->term_id,$child_cats_ids);
            unset($child_cats_ids[$key]);
            $counter++;
            break;
        }
    if( $counter >= 6 ) break; //max number of posts
    }
wp_reset_postdata();
}

?>