Hidden categories may have many subcategories. It is very common that all of the subcategories for a specific hidden category should be hidden too. I suggest to add a new magic word in addition to HIDDENCAT - for example DEEPHIDDENCAT, such that when a category is marked as a "deep hidden cat", all of its subcategories (no matter how deep) will become hidden too.

An example for this can be found in https://commons.wikimedia.org/wiki/Category:Photographs_by_Ermell - all of the subcategories really should be hidden, but only some of those are - and it's a very tedious maintenance work to put the HIDDENCAT magic word in all of those (I'm sure examples of more prominent categories can be found - but that's the one that I encountered upon).