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. Except, it's not very effective, because as Google notes Changes may take up to 24 hours to propagate to all users.