How to display grandchildren only of custom post?

Solved my own question after further research and testing 🙂 Ended up using 2 queries passing the result from one query to the other. Hope this helps someone.

    $argsIssues = array( 'post_type' => 'magazine',
        'post_parent__in' => array(20321), //get issue(children) posts from the issues(parent)
        'fields' => 'ids' //query only the postIDs
    );
    $q = get_posts( $argsIssues ); //run $argsIssues query

    $argsArticles = array( 'post_type' => 'magazine',
        'post_parent__in' => $q //get article(grandchildren) posts from issue(children) posts.
    );

    query_posts( $argsArticles ); //run $argsArticles query

    //while loop goes here to display posts from $argsArticles query

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