Thanks pointing me to the right table and field. This helps a lot !
So, I’ll do it with phpMyAdmin.
Oops! That’s not so easy. I tried something like this.
UPDATE modx_site_content
SET parent='20'
WHERE id IN(3,4,5,6,7)
Now, the moved documents do not display anymore in the document tree !
I assume there is some other table to update...
Who can help ?
... I see that I must also update the "isfolder" field (in table site_content) for documents that were made parents.
After setting the "isfolder" field of parent documents to "1", all works fine.