Codes explains that category__and will not pull in child categories
Multiple Category Handling
Display posts that are in multiple categories. This shows posts that are in both categories 2 and 6:
$query = new WP_Query( array( 'category__and' => array( 2, 6 ) ) );
To display posts from either category 2 OR 6, you could use cat as mentioned above, or by using category__in (note this does not show posts from any children of these categories):
One approach is select all 9763 posts using get_posts. This puts them in an array. Then loop through the array, selecting posts that are in 9751 or its child categories. (First make an array of all the child categories of 9751) It requires some custom code. I googled looking for an article for you to copy but didn't find one.