get_posts only children from certain parents

I have parent posts (custom post type hierarch=true) with ID 1, 2, 3, 4.

The parent posts with ID 2 and 4 have child pages.

I need to retrieve only posts 2 and 4 and all of their child pages.

Something like this

$argsch = array('orderby' => 'date','order' => 'DESC','post_type' => 'products', 'include' => '2, 4');
$childs = get_posts($argsch);

How can I edit this to return child pages of the included id’s?

You have two options:

1. Call get_posts multiple times: one for the parent pages using post__in => array(2,4), and two for the child pages of each parent with post_parent => 2 and post_parent => 4 and finally merge all the results into a single array.

2. Write directly your SQL query and use $wpdb->get_results. See this article for an example. In your case it would be something similar to the following (not tested) code:

   $query_sql = "
       SELECT *
       FROM $wpdb->posts
       WHERE post_type="products"
       AND (ID IN (2,4) OR post_parent IN (2,4))
       ORDER BY post_date DESC
   ";
   $query_result = $wpdb->get_results($query_sql, OBJECT);