post_parent array doesn’t work

It’s not recommended to use query_posts(), check for example this warning note in the Codex:

This function isn’t meant to be used by plugins or themes. As
explained later, there are better, more performant options to alter
the main query. query_posts() is overly simplistic and problematic way
to modify main query of a page by replacing it with new instance of
the query. It is inefficient (re-runs SQL queries) and will outright
fail in some circumstances (especially often when dealing with posts
pagination). Any modern WP code should use more reliable methods, like
making use of pre_get_posts hook, for this purpose.

Regarding the post parents array, you’re using a wrong parameter. There exists the post_parent__in parameter that can handle an array input.

So please try this secondary query, instead of your current one:

$query = new WP_Query( 
    array(
        'post_type'       => 'page', 
        'post_parent__in' => array(376, 379), 
        'orderby'         => 'title', 
        'order'           => ASC,  
        'posts_per_page'  => 9999
    )
);

You should also consider initializing your variable with $return_string = ''; to avoid the PHP Undefined variable notice when you use the .= for string concatenation.

Then replace wp_reset_query() with wp_reset_postdata(), to reset the $post variable of the main query.