To display all the posts that belong to a category and include multiple parentage (that is, display also the posts that belong to a meta-children term), we'll have to redo a bit the query of The Loop. What we'll do is retrieve all the children of the queried term, and use the list of ids for the category__in
parameter of the query. So, just before The Loop starts (normally it's with if( have_posts() ): ), just add the following:
$posts = query_posts( array( 'category__in' => get_all_meta_children( (int)get_query_var( 'cat' ) ) , 'paged' => get_query_var( 'paged' ) ) );
You can also set some parameters to the get_all_meta_children
function, such as the depth of children to display (that is, the "generations" of children and grandsons &c to include); and, actually, you don't need to pass the query var cat
parameter; it's just there for clarity.