Unless you are able to watch them every waking hour, this is simply not possible. At some point you have to cook, clean and generally do all the stuff that needs doing without worrying too much about your child hurting itself.

What generally happens is you set rules and teach what is acceptable or not. When you are not around the kid is expected to follow the rules.

Many kids do not follow these rules. They get punished and have valued things taken away. Some learn a lesson and will follow the rules and others do not.

The ones who still don't listen have to be with you when you cook and clean. If you go out another parent/babysitter/old children will watch them.

You’re missing the point, I believe.

What rules can you set in place such that your kids don’t accidentally see something explicitly targeting them with gore or other traumatic content?

The only rule that seems to make that possible is “don’t use YouTube Kids”.

Unless you are able to watch them every waking hour, there's nothing preventing your child from clicking on a video which is NOT labeled child-friendly.

It is not Youtube's responsibility to raise your children.