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 think the ethics mostly come into how you raise them, religion or not. It’s ethical to teach kindness and empathy. It’s ethical to allow your kids to explore while asking them questions that help that exploration. You can do those kinds of things no matter what faith (or non-faith) you practice.
Speaking as someone who was raised in an environment that gave lip service to kindness and empathy but was really very harsh, judgmental, and rigid, only one of my siblings kept something reasonably approximating my parents’ faith. The rest of us are mostly some variety of pagan. Each of us had a painful journey out of our parents’ faith to something. No matter how you raise your kids, they are their own people and will come to their own conclusions. You can make the path much more difficult than it needs to be or you can set them up for a much less traumatic journey.