Locking YouTube Account

My little boy is obsessed with YouTube and as he has Autism, we use it as a learning tool for him and to help calm him down.

But he has been watching really bad content, so we have now restricted his account and it is supervised via Google.

However the clever little git, just logs out and watches as a guest ! So he has no restrictions.

You tube and Google said it can be locked on android, but I would need to ask on here if it could be done on Apple??

Thanks Mark