Polite note: this thread is intended for parents who already allow their children to use YouTube kids. I would appreciate it if it didn't turn into a debate 
As I was loading up the YouTube kids app for my children yesterday, I saw, in small print, "YouTube kids is moving" at the bottom of the screen. When I clicked it, it said that they're integrating child profiles into the main YouTube app from July. I googled this to see if there was any other news/discussion but couldn't really find anything.
At present, we PIN protect the main YouTube app on our smart TV/fire stick/virgin box so that our children can't access YouTube unsupervised. When the integration happens, they won't be able to access YouTube kids without also being able to access YouTube. Even with restricted mode switched on, there's still a lot of stuff on there that I wouldn't want my children to see.
I spoke to Google via live chat yesterday to ask if there will be any PIN/password protection on "main" YouTube so that children can only access YouTube kids and they said there's no plans for it but it's a good idea and maybe it's something they could add in future. They said the best thing to do is to contact them through the "send feedback" option in the YouTube app. I have since done this.
If your children regularly use YouTube kids, I would suggest you also contact them about this. The lack of results on google search about it would suggest that many parents don't know this change is happening. I, perhaps optimistically, hope that the more requests they get, the more likely they are to listen and implement this. I wouldn't want anyone's children to accidentally stumble across unsuitable content - we all know how gross the internet can be! (I am aware that unsuitable content exists on YouTube kids too, but it's much less prevalent and easier to filter out!)
If you did want to contact them, you can find "send feedback" by clicking your profile picture in the YouTube app, going to "settings" and then "about."