luKkw

I managed to find the real problem of its disappearance (although it doesn’t solves the problem) :

It seems that there is a conflict between jquery libraries when I enable another jquery library from googleapis. The one used by the plugin (“jquery.tools.min”) is loaded but just doesn’t work.

So when I remove the google jquery library one, the plugin works again perfectly.

I tried few things found here and there, like this one , but without further knowledge of javascript, it seems I’m on a dead end

I managed to improve the code a little bit for those who may need a more that 1 level of hierarchy (a folder which include multiple folders which hold children files)
The previous one doesn’t seems to work beyond one level of parent-child relationship

This one does the trick for me

<?php
global $post;
$children = get_pages( array( 'child_of' => $post->ID, 'post_type' => 'MYCUSTOMPOSTTYPE' ) );
if ( is_single() && count( $children ) > 0 ) : ?>
<?php include STYLESHEETPATH . '/PARENT-TEMPLATE.php';//Load template for parent ?>
<?php else : ?>
<?php include STYLESHEETPATH . '/CHILD-TEMPLATE.php'; //Load template for child ?>
<?php endif; ?>

I’m pretty sure it is possible but you’ll have to change either the plugin code (which is highly above my skills) or use (and sacrifice) another wp-dtree-XXX.php (like the category one).
In this last solution, I suggest trying to copypaste the whole code of wp-dtree-pge.php to the one you’re willing to sacrifice. Unfortunately I do not remember if it is necessary to change something somewhere else.