If you are on a computer, you can open a new tab or window so you can listen to YouTube as you play. If on different windows, you may minimize the windows so you can have both screens on at the same time, though it will be distracting to have to try to watch two things at once. If you are on a phone, the YouTube app does not play when you are either on another app or with the screen off. There are two solutions to this:
1.) A subscription to YouTube Red will allow you to play youtube videos as you visit other apps or have the screen off. It is $9.99 per month, and also comes with other benefits.
2.) Play YouTube on another device while playing Kidblock.