How to only show posts on last child category

There is of course the include_children parameter for WP_Query as part of the taxonomy parameters. Which I suppose should work like this for you:

$args = array(
    'tax_query' => array(
        array(
            'include_children' => false
        ),
    ),
);
$query = new WP_Query( $args );

Or via parse_tax_query for your category archive:

add_filter( 
    'parse_tax_query', 
    'wpse163572_do_not_include_children_in_category_archive_parse_tax_query' 
);
function wpse163572_do_not_include_children_in_category_archive_parse_tax_query( $query ) {
    if ( 
        ! is_admin() 
        && $query->is_main_query()
        && $query->is_category()
    ) {
        // as seen here: http://wordpress.stackexchange.com/a/140952/22534
        $query->tax_query->queries[0]['include_children'] = 0;
    }
}

Note: Thanks to @PieterGoosen this is now tested and confirmed working.