How to Block Youtube on my mac chrome

How to do you block all access to YouTube in all browsers. My kids have to use Chrome for some school work, so Parental Controls doesn't cut it since it only blocks Safari. I can use extensions like Block Site for Chrome, but it doesn't block embedded videos.

How can I block something like YouTube for all browsers?

Ideally, we would like to be able to turn it off when the kids need access, then easily switch it back on. Is something like this possible?

Thanks!