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?
I’m not sure this is a question of ethics. It’s a question of whether you agree with a particular parent’s world view. A good parent tries to set their child on a positive path in life, and they are going to pick a path based their personal knowledge and beliefs.
Even if you try hard not to “indoctrinate” your child with any particular world view, they will still see you as a model for what to believe and how to behave. You will tend to be your child’s baseline for what “normal” is, at least early in their life. Your beliefs and behaviors will affect your kids whether you want them to or not.