I am concerned of the many queries, and my worst fear is the case that I would need an additional query in the loop, to get the last threads not only from forumid but from it and all its child-forums

aranoid:
Is there a way that I could modify the already used query to also include the subforums of $forumid ???
Code:
$getlast5posts=$DB_site->query("SELECT threadid,title FROM thread
WHERE forumid = $forumid
ORDER BY lastpost DESC limit 5");
I don't know much of sql, but of what I can see the Thread table has nothing relevant to "child forums" or a "parentid", which would be of some help. Any ideas ?
many thanks