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?
Ethically, depending on the religion, it is absolutely mandatory for parents to teach their children their religious views.
For example, let’s make up a cult. “Pireneists” are devout religious cultists that genuinely believe in their god, Kundo. Kundo’s holy book says that any who partake in the evil plant, the peanut, have been led astray by evil and will suffer for all eternity in the dark chasm of the lost.
For parents who legitimately believe this it would be completely unethical for them to let their children eat peanuts, their mental state has everything to do with their ethical mandates. The only ethical thing to do is to teach their children about their beliefs in such a way that the children will follow the same beliefs for their whole life. Indoctrination is indeed within the bounds of ethics.
To you it may seem silly. In fact to most of us this is peak idiocy and if the leaders of the pireneists have been known to take money from people to pay for their lavish lifestyles you could say that the organization itself is evil. However the mental state and beliefs of the parents override the fundamental veracity of the claims of the cult/religion. True or not, the parents believe and their inaction would be unethical.