Hi Guys, I actually managed to get my filter displaying single root categories, and only sub child categories, using the following. Seems to work great. Not sure how elegant it is, but it works.
SELECT 'DocumentCategoryIDs','-1', '(all)' AS CategoryNamePath
UNION SELECT '+DocumentCategoryIDs', CategoryID, CategoryDisplayName
FROM CMS_Category
WHERE ##WHERE##
ORDER BY ##ORDERBY##
Where Clause :
CategoryLevel > 1 AND CategoryParentID IN (21,31,34,38,42,50,54,57,60)