get_posts only children from certain parents

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);