Yes, give it a bit more time. It was not working for me earlier today.I
was in touch with Chromecast support and they said that the fix should
be rolled out to everyone by the end of the week.
I have good news: I just tried adding the child account my Chromecast 4
and it worked!The account was properly created and I was able to set it
up, including the child profile in YouTube.
I’m having the exact same issues over here with a Chromecast 4. It
started when I tried to login into to the child’s account on YouTube
after Google shut down YouTube Kids. Multiple failed attempts. Then
removed the child account and could not re add...