Thanks for the tip but i actually ended up adding the following at the top of the pages i needed it on :
$args = array(
'post_parent' => 0,
Not very DRY as it is repeated (i could have done a function though…) but it does the job perfectly.
HOWEVER, since every loop resets the $args array, i think it only works for me because my dropdown menu with the forums is before any other loop on the page.
Here is the code i added in the pages i needed to show the dropdown (bootstrap):