For many religious people, raising their children in their faith is an important part of their religious practice. They might see getting their kids into heaven as one of the most important things they can do as parent. And certainly, adults should have the right to practice their religion freely, but children are impressionable and unlikely to realize that they are being indoctrinated into one religion out of the thousands that humans practice.
And many faith traditions have beliefs that are at odds with science or support bigoted worldviews. For example, a queer person being raised in the Catholic Church would be taught that they are inherently disordered and would likely be discouraged from being involved in LGBTQ support groups.
Where do you think the line is between practicing your own religion faithfully and unethically forcing your beliefs on someone else?
If a child is homosexual, I would argue its unethical to teach them they are freak of nature and they are wrong or broken. However, its not illegal.
It’s act vs rule ethics, what is ethical in a particular situation may not be broadly applicable to society.
Edit: And from the religious parents perspective, letting your beloved child suffer an eternity of torment is probably not super moral. I may disagree but that’s their perspective and there’s no arbiter make the call.
You’re citing Bentham Utilitarianism but you could make a stronger argument for your side if you cited Kant I would think.
Utilitarianism makes sense from first principles, Kant is just his opinions.