Whitelisting: There is way too much appropriate content out there to whitelist it all. It's totally infeasible for a parent, unless you're planning to only approve a handful of channels, which makes YouTube pointless.
YouTube Kids: Teenagers are not "kids" and are not going to go onto YouTube Kids to watch Baby Shark and Mickey Mouse Clubhouse or whatever other kiddie stuff they have there.
Something else entirely is needed here.
edit: Oh neat they do have a parental approval mode in there now. Last time I was in here they only let you set an age range for the content that you wanted. It still seems a bit weird though, I can select a channel from the list they are presenting me but I can't search for some arbitrary channel to unlock. I'll have another look tonight though
The UX is a complete fail, my comment having a moan about it all has over 90 upvotes now, yet I'm wrong. But the reason I'm wrong is because they've made it painfully hard to manage.
Kind of weird that there are so many comments here lamenting the lack of this feature when it actually exists just as requested.