Parental Controls: Trying to block YouTube

I've enabled Parental Controls for my granddaughter's account on my iMac, running High Sierra 10.13.3. It seems to work fine for blocking applications I don't want her to access. But for websites, we are choosing the option "Try to limit access..." and through the "Customize..." button have added https://www.youtube.com to the list of "Never allow these websites:". It isn't working as it is still possible to access that website.

My only option that I can see is to use the "Allow access to only these websites" and list those sites, but that is too restrictive and I don't feel like adding the 100 or so sites that are ok for her to access and constantly managing that list.

I really want the ability to just list the sites we don't want her to access instead. Anyone else having this problem? Can someone suggest a fix?