Thanks Ivovic.
The plugin does exist and is named phpmyadmin, found it on Google easily.
When installed I found the categories parent/child list by first browsing to the wordpress database, then to wp_term_taxonomy (you’ll find it on the left) and there it is possible to reset all child-categories to no-parent.
And like you said, they showed up in the wp interfaces.
Thanks again for the reply.
Hope this can help someone else as well.
