http://e26whn2524322mkxb3cbyk27ev2ihhq2biz35hty7gzgsyrwrygq27yd.onion/posts/blog/the-internet/attempting-to-control-youtube-access-on-android.html
Going through that menu, you get two choices ON for everyone OFF for everyone which isn't what I want. You can't turn on/off for just a single user, but can for a group, so I created a group called "Youtube allowed", enabled Youtube for that group, and then disabled Youtube for everyone else.