Is there any evidence for that?
Do we need empirical evidence to warrant thinking that telling children that people suffer torment in hell forever is harmful and bad? I don’t think so. There are things that we know without evidence. For instance we know that telling people they are stupid or ugly or boring or generally repulsive is bad. We also know that bad news is bad, so we know that it’s bad to tell people bad news if it’s not true – we know it’s bad to tell someone: ‘your cat/dog/best friend/mother/child is injured and in terrible pain’ if that’s not true.
We don’t need evidence for that. It’s part of how the world is. Imagine telling a child: ‘Your cat is caught in a trap, it’s crushing her leg in its jaws, she’s howling in pain, we can’t get her out’ when it’s not true. There’s no way to look at that and think it’s good or not too bad or neutral. Even if we knew for a fact that it would do no lasting psychic damage at all (and how would we know that?), it would still be bad. Even temporary mental anguish is bad.
We don’t demand research before we refrain from doing things like that. We don’t, and don’t need to, and shouldn’t. We extrapolate – from experience, imagination, sympathy, empathy. We know what that would feel like, and we flinch, and we don’t do it to people.
That’s how a lot of morality works, at the simplest level. That’s why one familiar parental sqawk is ‘How would you like it if she did that to you?’ It’s the most direct way to explain why something is wrong and not permitted. The child being squawked at doesn’t get to demand a look at the research before accepting the lesson.
So – adults who tell children there is a hell where some people are tortured forever are doing a bad thing, even if the children do simply ignore the claim, or shrug it off, or deny it. If the children believe it but think it is only other people who are tortured forever and are happy with that thought – that is a very bad thing, because those are some callous children, if not outright sadistic.