Is it fine to tell the kids that they are 'bad' when they really are? Or would you go for telling them that "this behavior is not acceptable" when they misbehave?
I would go for telling them that it is "bad" and explain why it is bad. When you say "it is not acceptable", they would ask why and you'd only end up telling them that's because it is bad and why it is bad.
Yeah, you don't want them to get low self esteems by making them think they are bad. You want them to identify the behavior as bad so you can help them change it.