Telling a child that they are being watched 24/7 by a being who will torture them forever in the harshest way possible for thinking or doing certain things is child abuse. What's worse is teaching children to adore the very being who will do this to them.
It is also unethical, and ultimately child abuse to teach a child to value a fictional book that promotes rape, sexism, mass murder, torture in all forms, racism, and overall hate.
You know what isn't child abuse? Teaching your children about the amazing and complex science behind how the earth was created, and how humans evolved, and not shoving false blasphemy down the throat of a young person.
What sort of "parent" would ever tell their child that they are so bad that they deserve to be tortured forever simply for not doing whatever their parents' holy man tells them to? What if we had parents who told their child that they would be tortured in hellfire forever for not showing up to school on time or for forgetting to walk the dog? Would THAT be abuse? I worry for anyone who would say no.
A child's mind is far too malleable. Though @Midnight1131 states an atheist would realise that hell doesn't exist and thus wouldn't care about being threatened, this would only apply to an adult or matured child. However, a young child that is threatened, especially by their parents, is going to be considerably impacted. Though they might not believe their parents, the fact that they'd feel rejected or disliked by their own parents is enough to cause considerable mental damage, and result in both major and minor defects.
That's just dumb we all no if your atheist then hell awaits it just fact. Also who cares. Is mental abuse even a thing, if it is why that just sounds really stupid, like physical abuse makes sense but mental, not so much. What even falls under the category of mental abuse, that's just dumb.
I think that threatening an atheist with hell isn't a real threat, because hell doesn't actually exist, and an atheist realizes that. When they're being threatened with torture coming from a "so called God who loves them," it will only strengthen their resolve that God is only a fabricated story.