I think you can only whitelist channels that are approved kids channels, not from the full youtube library, but it does allow you to avoid the ocean of crap and pick just the good stuff.
You can actually share any channel!
It is a very hidden feature though. You have to link your kids account to yours, and then in the Youtube mobile app (not on the web), you can click the three dots button and then share > "with kids"
So that way you can build up a whitelist of channels that they can see in the YT kids account.